fix(stats): subtract lifetime totals incrementally on delete (#196)

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

@@ -555,6 +555,16 @@ function ensureAnimeMergeTables(db: DatabaseSync): void {
   `);
 }
 
+function ensureSubtitleLineEventIndex(db: DatabaseSync): void {
+  // The event_id FK is ON DELETE SET NULL; without this index every deleted
+  // imm_session_events row full-scans imm_subtitle_lines looking for children,
+  // which made session deletes take minutes on large databases.
+  db.exec(`
+    CREATE INDEX IF NOT EXISTS idx_subtitle_lines_event_id
+    ON imm_subtitle_lines(event_id) WHERE event_id IS NOT NULL
+  `);
+}
+
 export function getOrCreateAnimeRecord(db: DatabaseSync, input: AnimeRecordInput): number {
   const seasonScope = normalizeSeasonScope(input.seasonScope);
   const identityTitle = buildSeasonScopedAnimeTitle(input.parsedTitle, seasonScope);
@@ -888,6 +898,7 @@ export function ensureSchema(db: DatabaseSync): void {
     ensureLifetimeSummaryTables(db);
     ensureStatsExcludedWordsTable(db);
     ensureAnimeMergeTables(db);
+    ensureSubtitleLineEventIndex(db);
     return;
   }
 
@@ -1525,6 +1536,7 @@ export function ensureSchema(db: DatabaseSync): void {
     CREATE INDEX IF NOT EXISTS idx_subtitle_lines_anime_line
     ON imm_subtitle_lines(anime_id, line_index)
   `);
+  ensureSubtitleLineEventIndex(db);
   if (currentVersion?.schema_version && currentVersion.schema_version < 19) {
     backfillLexicalOccurrenceSeenMs(db);
   }