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.test.ts

@@ -46,9 +46,16 @@ test('extractSubtitleTrackToFile downloads an external track served over http',
   const server = await startSubtitleServer();
   try {
     const result = await extractSubtitleTrackToFile({
-      ffmpegPath: '/unused/ffmpeg',
+      resolveFfmpegPath: () => {
+        throw new Error('external tracks must not resolve ffmpeg');
+      },
       videoPath: server.url('/stream.mp4'),
-      track: { id: 2, type: 'sub', external: true, 'external-filename': server.url('/subs/ja.srt') },
+      track: {
+        id: 2,
+        type: 'sub',
+        external: true,
+        'external-filename': server.url('/subs/ja.srt'),
+      },
       httpHeaders: null,
     });
 
@@ -72,9 +79,16 @@ test('extractSubtitleTrackToFile forwards mpv request headers to the subtitle ho
 
   try {
     const result = await extractSubtitleTrackToFile({
-      ffmpegPath: '/unused/ffmpeg',
+      resolveFfmpegPath: () => {
+        throw new Error('external tracks must not resolve ffmpeg');
+      },
       videoPath: server.url('/stream.mp4'),
-      track: { id: 2, type: 'sub', external: true, 'external-filename': server.url('/subs/ja.srt') },
+      track: {
+        id: 2,
+        type: 'sub',
+        external: true,
+        'external-filename': server.url('/subs/ja.srt'),
+      },
       httpHeaders,
     });
     cleanupTemporaryFile(result);
@@ -92,7 +106,9 @@ test('extractSubtitleTrackToFile reports the HTTP status when the download fails
   try {
     await assert.rejects(
       extractSubtitleTrackToFile({
-        ffmpegPath: '/unused/ffmpeg',
+        resolveFfmpegPath: () => {
+          throw new Error('external tracks must not resolve ffmpeg');
+        },
         videoPath: server.url('/stream.mp4'),
         track: {
           id: 2,
@@ -116,7 +132,9 @@ test('extractSubtitleTrackToFile still uses a local external track in place', as
 
   try {
     const result = await extractSubtitleTrackToFile({
-      ffmpegPath: '/unused/ffmpeg',
+      resolveFfmpegPath: () => {
+        throw new Error('external tracks must not resolve ffmpeg');
+      },
       videoPath: path.join(dir, 'video.mkv'),
       track: { id: 2, type: 'sub', external: true, 'external-filename': localPath },
       httpHeaders: null,
@@ -134,7 +152,9 @@ test('extractSubtitleTrackToFile still uses a local external track in place', as
 test('extractSubtitleTrackToFile rejects a missing local external track', async () => {
   await assert.rejects(
     extractSubtitleTrackToFile({
-      ffmpegPath: '/unused/ffmpeg',
+      resolveFfmpegPath: () => {
+        throw new Error('external tracks must not resolve ffmpeg');
+      },
       videoPath: '/tmp/video.mkv',
       track: { id: 2, type: 'sub', external: true, 'external-filename': '/tmp/does-not-exist.srt' },
       httpHeaders: null,