fix(subsync): resolve ffmpeg only when a track needs extraction

Stream subtitles arrive as URLs and are downloaded, never piped through ffmpeg, so demanding an ffmpeg install up front broke stream syncs (and CI, which has no ffmpeg). Resolution is now lazy, scoped to embedded tracks.

src/core/services/subsync-extract.ts

@@ -24,7 +24,11 @@ export interface FileExtractionResult {
 }
 
 export interface SubtitleExtractionInput {
-  ffmpegPath: string;
+  /**
+   * Resolved lazily: an external track never shells out to ffmpeg, so a stream
+   * whose subtitles arrive by URL must not fail just because ffmpeg is absent.
+   */
+  resolveFfmpegPath: () => string;
   videoPath: string;
   track: MpvTrack;
   /** mpv's request context, so stream-hosted tracks stay reachable. */
@@ -41,6 +45,19 @@ function extensionForUrl(url: string, fallback: string): string {
   }
 }
 
+/**
+ * Drop a scratch directory whose extraction never completed.
+ *
+ * Removal is recursive here — unlike `cleanupTemporaryFile`, nothing in a failed
+ * directory is worth keeping, and a half-written download would otherwise leave
+ * the directory behind for the life of the machine.
+ */
+function discardTemporaryDirectory(tempDir: string): void {
+  try {
+    fs.rmSync(tempDir, { recursive: true, force: true });
+  } catch {}
+}
+
 /**
  * Pull a subtitle track mpv loaded from a URL down to disk.
  *
@@ -53,18 +70,25 @@ async function downloadRemoteSubtitleTrack(
   httpHeaders: ResolvedMpvHttpHeaders | null,
 ): Promise<FileExtractionResult> {
   const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'subminer-subsync-'));
-  const outputPath = path.join(tempDir, `remote_track.${extensionForUrl(url, 'srt')}`);
+  try {
+    const outputPath = path.join(tempDir, `remote_track.${extensionForUrl(url, 'srt')}`);
 
-  const result = await downloadToFile(url, outputPath, toRequestHeaders(httpHeaders));
-  if (!result.ok) {
-    throw new Error(`Failed to download subtitle track: ${result.error?.error ?? 'unknown error'}`);
-  }
-  if (!fileExists(outputPath)) {
-    throw new Error(`Downloaded subtitle track is missing: ${url}`);
-  }
+    const result = await downloadToFile(url, outputPath, toRequestHeaders(httpHeaders));
+    if (!result.ok) {
+      throw new Error(
+        `Failed to download subtitle track: ${result.error?.error ?? 'unknown error'}`,
+      );
+    }
+    if (!fileExists(outputPath)) {
+      throw new Error(`Downloaded subtitle track is missing: ${url}`);
+    }
 
-  logger.info(`Downloaded remote subtitle track to ${outputPath}`);
-  return { path: outputPath, temporary: true };
+    logger.info(`Downloaded remote subtitle track to ${outputPath}`);
+    return { path: outputPath, temporary: true };
+  } catch (error) {
+    discardTemporaryDirectory(tempDir);
+    throw error;
+  }
 }
 
 async function extractInternalTrack(input: SubtitleExtractionInput): Promise<FileExtractionResult> {
@@ -78,40 +102,45 @@ async function extractInternalTrack(input: SubtitleExtractionInput): Promise<Fil
   }
 
   const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'subminer-subsync-'));
-  const outputPath = path.join(tempDir, `track_${ffIndex}.${extension}`);
-  // Header args configure the HTTP demuxer, so they belong before `-i`.
-  const httpArgs = isRemoteMediaPath(input.videoPath)
-    ? toFfmpegInputHttpArgs(input.httpHeaders)
-    : [];
+  try {
+    const outputPath = path.join(tempDir, `track_${ffIndex}.${extension}`);
+    // Header args configure the HTTP demuxer, so they belong before `-i`.
+    const httpArgs = isRemoteMediaPath(input.videoPath)
+      ? toFfmpegInputHttpArgs(input.httpHeaders)
+      : [];
 
-  const extraction = await runCommand(input.ffmpegPath, [
-    '-hide_banner',
-    '-nostdin',
-    '-y',
-    '-loglevel',
-    'error',
-    ...httpArgs,
-    '-an',
-    '-vn',
-    '-i',
-    input.videoPath,
-    '-map',
-    `0:${ffIndex}`,
-    '-f',
-    extension,
-    outputPath,
-  ]);
+    const extraction = await runCommand(input.resolveFfmpegPath(), [
+      '-hide_banner',
+      '-nostdin',
+      '-y',
+      '-loglevel',
+      'error',
+      ...httpArgs,
+      '-an',
+      '-vn',
+      '-i',
+      input.videoPath,
+      '-map',
+      `0:${ffIndex}`,
+      '-f',
+      extension,
+      outputPath,
+    ]);
 
-  if (!extraction.ok || !fileExists(outputPath)) {
-    throw new Error(
-      `Failed to extract internal subtitle track with ffmpeg: ${summarizeCommandFailure(
-        'ffmpeg',
-        extraction,
-      )}`,
-    );
+    if (!extraction.ok || !fileExists(outputPath)) {
+      throw new Error(
+        `Failed to extract internal subtitle track with ffmpeg: ${summarizeCommandFailure(
+          'ffmpeg',
+          extraction,
+        )}`,
+      );
+    }
+
+    return { path: outputPath, temporary: true };
+  } catch (error) {
+    discardTemporaryDirectory(tempDir);
+    throw error;
   }
-
-  return { path: outputPath, temporary: true };
 }
 
 export async function extractSubtitleTrackToFile(