fix(stats): fix data loss and error handling in anime merge/move

- flush pending telemetry before merge/move so in-progress session time isn't dropped from lifetime totals
- repoint subtitle lines by video_id instead of anime_id so lines recorded before the async title parse assigns a link aren't stranded
- stop absorbing metadata into the target when moving an episode out of an emptied entry (a move isn't a same-show claim)
- return 404 only for missing episode/target, not storage failures; return 404 when a merge folds nothing
- keep AnimeMergeDialog open while a merge is in flight instead of letting dismiss race the request
- surface library load failures in LibraryEntryPicker instead of showing an empty list

src/core/services/__tests__/stats-server.test.ts

@@ -3109,6 +3109,26 @@ Aligned English subtitle
     });
   });
 
+  it('POST /api/stats/anime/:animeId/merge reports a merge that folded nothing as 404', async () => {
+    const app = createStatsApp(
+      createMockTracker({
+        mergeAnime: async (targetAnimeId: number) => ({
+          survivingAnimeId: targetAnimeId,
+          mergedAnimeIds: [],
+          movedVideos: 0,
+        }),
+      } as Partial<ImmersionTrackerService>),
+    );
+
+    const res = await app.request('/api/stats/anime/7/merge', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: '{"sourceAnimeIds":[8]}',
+    });
+
+    assert.equal(res.status, 404);
+  });
+
   it('PATCH /api/stats/media/:videoId/anime reports an unknown target as 404', async () => {
     const app = createStatsApp(
       createMockTracker({
@@ -3127,6 +3147,25 @@ Aligned English subtitle
     assert.equal(res.status, 404);
   });
 
+  it('PATCH /api/stats/media/:videoId/anime does not disguise storage failures as 404', async () => {
+    const app = createStatsApp(
+      createMockTracker({
+        moveVideoToAnime: async () => {
+          throw new Error('database is locked');
+        },
+      } as Partial<ImmersionTrackerService>),
+    );
+
+    const res = await app.request('/api/stats/media/12/anime', {
+      method: 'PATCH',
+      headers: { 'Content-Type': 'application/json' },
+      body: '{"animeId":7}',
+    });
+
+    assert.notEqual(res.status, 404);
+    assert.equal(res.status >= 500, true);
+  });
+
   it('POST /api/stats/anki/browse returns 400 for missing noteId', async () => {
     const app = createStatsApp(createMockTracker());
     const res = await app.request('/api/stats/anki/browse', { method: 'POST' });

src/core/services/immersion-tracker-service.ts

@@ -772,11 +772,18 @@ export class ImmersionTrackerService {
     if (pendingVideoId !== undefined) {
       await this.pendingAnimeMetadataUpdates.get(pendingVideoId);
     }
+    // Both of these rebuild the lifetime summaries, which recompute from the
+    // database: queued telemetry has to land first or the active session's
+    // watch time is dropped from the merged totals.
+    this.flushTelemetry(true);
+    this.flushNow();
     return mergeAnimeRecords(this.db, targetAnimeId, sourceAnimeIds);
   }
 
   async moveVideoToAnime(videoId: number, targetAnimeId: number): Promise<VideoMoveSummary> {
     await this.pendingAnimeMetadataUpdates.get(videoId);
+    this.flushTelemetry(true);
+    this.flushNow();
     return moveVideoToAnimeQuery(this.db, videoId, targetAnimeId);
   }
 

src/core/services/immersion-tracker/__tests__/anime-merge.test.ts

@@ -157,6 +157,33 @@ test('mergeAnimeRecords folds episodes, lines and lifetime totals into the targe
   });
 });
 
+test('mergeAnimeRecords repoints subtitle lines recorded before the anime link landed', () => {
+  withDb((db) => {
+    insertAnime(db, { animeId: 1, key: 'show', title: 'Show' });
+    insertAnime(db, { animeId: 2, key: 'show season 1', title: 'Show Season 1' });
+    insertEpisode(db, { videoId: 1, animeId: 1 });
+    insertEpisode(db, { videoId: 2, animeId: 2, season: 1 });
+    // Lines are written with the video's anime_id at the time, which is NULL
+    // until the async title parse assigns one.
+    db.prepare(
+      `INSERT INTO imm_subtitle_lines(session_id, video_id, anime_id, line_index, text, CREATED_DATE, LAST_UPDATE_DATE)
+       VALUES (2, 2, NULL, 2, 'unlinked line', ?, ?)`,
+    ).run(BASE_MS, BASE_MS);
+
+    mergeAnimeRecords(db, 1, [2]);
+
+    assert.equal(lineAnimeIds(db, 1), 3);
+    const orphaned = Number(
+      (
+        db
+          .prepare('SELECT COUNT(*) AS total FROM imm_subtitle_lines WHERE anime_id IS NULL')
+          .get() as { total: number }
+      ).total,
+    );
+    assert.equal(orphaned, 0);
+  });
+});
+
 test('mergeAnimeRecords inherits metadata the target is missing without clobbering its own', () => {
   withDb((db) => {
     insertAnime(db, { animeId: 1, key: 'show', title: 'Show', titleRomaji: 'Shou' });
@@ -215,6 +242,27 @@ test('moveVideoToAnime moves one episode and prunes the emptied entry', () => {
       .prepare('SELECT total_active_ms AS activeMs FROM imm_lifetime_anime WHERE anime_id = 1')
       .get() as { activeMs: number };
     assert.equal(lifetime.activeMs, 6000);
+    // The stray entry's AniList link is dropped, not inherited: a move makes no
+    // claim that the two entries are the same show.
+    const target = db
+      .prepare('SELECT anilist_id AS anilistId FROM imm_anime WHERE anime_id = 1')
+      .get() as { anilistId: number | null };
+    assert.equal(target.anilistId, null);
+  });
+});
+
+test('moveVideoToAnime is a no-op when the episode is already in the target entry', () => {
+  withDb((db) => {
+    insertAnime(db, { animeId: 1, key: 'show', title: 'Show' });
+    insertEpisode(db, { videoId: 1, animeId: 1 });
+
+    const summary = moveVideoToAnime(db, 1, 1);
+
+    assert.equal(summary.targetAnimeId, 1);
+    assert.equal(summary.previousAnimeId, 1);
+    assert.equal(summary.removedPreviousAnime, false);
+    assert.deepEqual(animeIds(db), [1]);
+    assert.equal(videoAnimeId(db, 1), 1);
   });
 });
 

src/core/services/immersion-tracker/anime-merge.ts

@@ -3,6 +3,9 @@ import { rebuildLifetimeSummariesInTransaction } from './lifetime';
 import { toDbTimestamp } from './query-shared';
 import { nowMs } from './time';
 
+/** Thrown when a move names an episode or destination entry that is not there. */
+export const UNKNOWN_MOVE_TARGET_MESSAGE = 'Unknown episode or target library entry';
+
 export interface AnimeMergeSummary {
   /** Library entry that owns every moved episode once the merge finishes. */
   survivingAnimeId: number;
@@ -161,11 +164,17 @@ export function mergeAnimeRecordsInTransaction(
   }
 
   const updatedAt = toDbTimestamp(nowMs());
+  const sourceVideosStmt = db.prepare(
+    'SELECT video_id AS videoId FROM imm_videos WHERE anime_id = ?',
+  );
   const moveVideosStmt = db.prepare(
     'UPDATE imm_videos SET anime_id = ?, LAST_UPDATE_DATE = ? WHERE anime_id = ?',
   );
+  // Repointed per video rather than by anime_id: lines recorded before the
+  // async title parse assigns the link are stored with a NULL anime_id, and
+  // matching on the source id would strand them unattributed.
   const moveLinesStmt = db.prepare(
-    'UPDATE imm_subtitle_lines SET anime_id = ?, LAST_UPDATE_DATE = ? WHERE anime_id = ?',
+    'UPDATE imm_subtitle_lines SET anime_id = ?, LAST_UPDATE_DATE = ? WHERE video_id = ?',
   );
   const dropLifetimeStmt = db.prepare('DELETE FROM imm_lifetime_anime WHERE anime_id = ?');
   const dropAnimeStmt = db.prepare('DELETE FROM imm_anime WHERE anime_id = ?');
@@ -176,10 +185,15 @@ export function mergeAnimeRecordsInTransaction(
     }
 
     const sourceMetadata = readAnimeMetadata(db, sourceAnimeId);
+    const sourceVideoIds = (sourceVideosStmt.all(sourceAnimeId) as Array<{ videoId: number }>).map(
+      (row) => row.videoId,
+    );
     const moved = moveVideosStmt.run(targetAnimeId, updatedAt, sourceAnimeId) as {
       changes: number;
     };
-    moveLinesStmt.run(targetAnimeId, updatedAt, sourceAnimeId);
+    for (const videoId of sourceVideoIds) {
+      moveLinesStmt.run(targetAnimeId, updatedAt, videoId);
+    }
     dropLifetimeStmt.run(sourceAnimeId);
     dropAnimeStmt.run(sourceAnimeId);
     absorbAnimeMetadata(db, targetAnimeId, sourceMetadata, updatedAt);
@@ -219,7 +233,7 @@ export function moveVideoToAnime(
       .prepare('SELECT anime_id AS animeId FROM imm_videos WHERE video_id = ?')
       .get(videoId) as { animeId: number | null } | null;
     if (!videoRow || !animeExists(db, targetAnimeId)) {
-      throw new Error('Unknown episode or target library entry');
+      throw new Error(UNKNOWN_MOVE_TARGET_MESSAGE);
     }
 
     const previousAnimeId = videoRow.animeId;
@@ -239,10 +253,12 @@ export function moveVideoToAnime(
 
     let removedPreviousAnime = false;
     if (previousAnimeId !== null && !hasAnimeReferences(db, previousAnimeId)) {
-      const sourceMetadata = readAnimeMetadata(db, previousAnimeId);
+      // The emptied entry's metadata is deliberately dropped rather than
+      // absorbed. A move says "this episode belongs elsewhere", not "these are
+      // the same show", and the entry being emptied is usually a mis-parse
+      // whose AniList link would be wrong for the target.
       db.prepare('DELETE FROM imm_lifetime_anime WHERE anime_id = ?').run(previousAnimeId);
       db.prepare('DELETE FROM imm_anime WHERE anime_id = ?').run(previousAnimeId);
-      absorbAnimeMetadata(db, targetAnimeId, sourceMetadata, updatedAt);
       removedPreviousAnime = true;
     }
 

src/core/services/immersion-tracker/anime-season-repair.ts

@@ -360,9 +360,11 @@ export function resolveAnimeAnilistConflict(
       const summary = emptySummary(1);
       summary.movedVideos = merge.movedVideos;
       summary.deletedAnimeRows = merge.mergedAnimeIds.length;
-      summary.survivingAnimeId = survivingAnimeId;
       if (merge.mergedAnimeIds.length > 0) {
         summary.repaired = 1;
+        // Only reported once a row really absorbed the other, so callers never
+        // follow this to an anime id that was never written.
+        summary.survivingAnimeId = survivingAnimeId;
       }
       // Lifetime summaries are rebuilt by the caller off this summary, the same
       // as the redistribution path below.
@@ -383,11 +385,16 @@ function canMergeAnilistConflict(
   anilistId: number,
   options: AnimeAnilistConflictOptions,
 ): boolean {
+  const targetRow = getAnimeRow(db, targetAnimeId);
+  if (!targetRow) {
+    // Nothing to merge with a row that no longer exists (a stale id from the
+    // caller); fall through to the redistribution path.
+    return false;
+  }
   if (options.survivor !== 'target') {
     // The target is the row about to disappear here, so an existing link of its
     // own means this is a mis-resolution rather than a duplicate: leave it be.
-    const targetRow = getAnimeRow(db, targetAnimeId);
+    if (targetRow.anilist_id != null && targetRow.anilist_id !== anilistId) {
-    if (targetRow?.anilist_id != null && targetRow.anilist_id !== anilistId) {
       return false;
     }
   }

src/core/services/stats-server/library-routes.ts

@@ -1,5 +1,6 @@
 import type { Hono } from 'hono';
 import { statsJson } from '../../../types/stats-http-contract.js';
+import { UNKNOWN_MOVE_TARGET_MESSAGE } from '../immersion-tracker/anime-merge.js';
 import type { ImmersionTrackerService } from '../immersion-tracker-service.js';
 import {
   buildSentenceSearchOptions,
@@ -206,6 +207,9 @@ export function registerStatsLibraryRoutes(
     const sourceAnimeIds = parsePositiveIdList(body?.sourceAnimeIds).filter((id) => id !== animeId);
     if (sourceAnimeIds.length === 0) return c.body(null, 400);
     const summary = await tracker.mergeAnime(animeId, sourceAnimeIds);
+    // Nothing folded means the target or every source was already gone, so the
+    // caller should not be told the merge succeeded.
+    if (summary.mergedAnimeIds.length === 0) return c.body(null, 404);
     return c.json(
       statsJson('mergeAnime', {
         ok: true,
@@ -232,8 +236,13 @@ export function registerStatsLibraryRoutes(
           removedPreviousAnime: summary.removedPreviousAnime,
         }),
       );
-    } catch {
+    } catch (error) {
-      return c.body(null, 404);
+      // Only a missing episode or entry is a 404; storage failures must not be
+      // reported to the caller as "not found".
+      if (error instanceof Error && error.message === UNKNOWN_MOVE_TARGET_MESSAGE) {
+        return c.body(null, 404);
+      }
+      throw error;
     }
   });
 }

stats/src/components/anime/AnimeMergeDialog.tsx

@@ -43,8 +43,18 @@ export function AnimeMergeDialog({ entries, onClose, onMerged }: AnimeMergeDialo
     }
   };
 
+  // Dismissing mid-request would leave the caller unaware of a merge that is
+  // still going to land, so the backdrop and close button are inert until it
+  // resolves.
+  const handleDismiss = () => {
+    if (!merging) onClose();
+  };
+
   return (
-    <div className="fixed inset-0 z-50 flex items-start justify-center pt-[10vh]" onClick={onClose}>
+    <div
+      className="fixed inset-0 z-50 flex items-start justify-center pt-[10vh]"
+      onClick={handleDismiss}
+    >
       <div className="absolute inset-0 bg-ctp-crust/70 backdrop-blur-[2px]" />
       <div
         className="relative bg-ctp-base border border-ctp-surface1 rounded-xl shadow-2xl w-full max-w-lg max-h-[70vh] flex flex-col animate-fade-in"
@@ -57,8 +67,9 @@ export function AnimeMergeDialog({ entries, onClose, onMerged }: AnimeMergeDialo
             </h3>
             <button
               type="button"
-              onClick={onClose}
+              onClick={handleDismiss}
-              className="text-ctp-overlay2 hover:text-ctp-text text-lg leading-none"
+              disabled={merging}
+              className="text-ctp-overlay2 hover:text-ctp-text text-lg leading-none disabled:opacity-50"
             >
               {'✕'}
             </button>

stats/src/components/anime/EpisodeList.tsx

@@ -205,7 +205,7 @@ export function EpisodeList({
                             setMoveError(null);
                             setMovingEpisode(ep);
                           }}
-                          className={`w-5 h-5 rounded border border-ctp-surface2 text-transparent hover:border-ctp-blue/50 hover:text-ctp-blue hover:bg-ctp-blue/10 transition-colors text-xs flex items-center justify-center ${HOVER_REVEALED}`}
+                          className={`w-5 h-5 rounded border border-ctp-surface2 text-transparent hover:border-ctp-blue/50 hover:text-ctp-blue focus-visible:text-ctp-blue hover:bg-ctp-blue/10 transition-colors text-xs flex items-center justify-center ${HOVER_REVEALED}`}
                           title="Move to another library entry"
                           aria-label="Move to another library entry"
                         >
@@ -217,7 +217,7 @@ export function EpisodeList({
                             e.stopPropagation();
                             void handleDeleteEpisode(ep.videoId, ep.canonicalTitle);
                           }}
-                          className={`w-5 h-5 rounded border border-ctp-surface2 text-transparent hover:border-ctp-red/50 hover:text-ctp-red hover:bg-ctp-red/10 transition-colors text-xs flex items-center justify-center ${HOVER_REVEALED}`}
+                          className={`w-5 h-5 rounded border border-ctp-surface2 text-transparent hover:border-ctp-red/50 hover:text-ctp-red focus-visible:text-ctp-red hover:bg-ctp-red/10 transition-colors text-xs flex items-center justify-center ${HOVER_REVEALED}`}
                           title="Delete episode"
                           aria-label="Delete episode"
                         >

stats/src/components/anime/LibraryEntryPicker.tsx

@@ -25,6 +25,7 @@ export function LibraryEntryPicker({
   onClose,
 }: LibraryEntryPickerProps) {
   const [entries, setEntries] = useState<AnimeLibraryItem[] | null>(null);
+  const [loadFailed, setLoadFailed] = useState(false);
   const [query, setQuery] = useState(initialQuery);
   const inputRef = useRef<HTMLInputElement>(null);
 
@@ -37,7 +38,11 @@ export function LibraryEntryPicker({
         if (!cancelled) setEntries(data);
       })
       .catch(() => {
-        if (!cancelled) setEntries([]);
+        // Distinct from an empty library: telling the user "no other titles"
+        // when the request failed hides a retryable error.
+        if (cancelled) return;
+        setEntries([]);
+        setLoadFailed(true);
       });
     return () => {
       cancelled = true;
@@ -84,7 +89,12 @@ export function LibraryEntryPicker({
 
         <div className="flex-1 overflow-y-auto p-2">
           {entries === null && <div className="text-xs text-ctp-overlay2 p-3">Loading...</div>}
-          {entries !== null && visible.length === 0 && (
+          {loadFailed && (
+            <div className="text-xs text-ctp-red p-3">
+              Could not load the library. Close this dialog and try again.
+            </div>
+          )}
+          {!loadFailed && entries !== null && visible.length === 0 && (
             <div className="text-xs text-ctp-overlay2 p-3">No other titles</div>
           )}
           {visible.map((entry) => (