feat(transcode): optimize + on-the-fly quality + HLS streaming (§6.6)

POST /tracks/{id}/optimize enqueues a transcode; GET /stream/{id}?quality=
serves a cached Opus rendition (miss → master + background warm); GET
/stream/{id}/hls/{playlist.m3u8,segment} serves the worker-generated HLS
rendition (AAC-in-TS), with ?token= propagated onto segment URLs for players
that can't set headers. Hexagonal: Transcoder port, FfmpegTranscoder adapter,
TranscodeService + cache-path helpers, transcode_track worker (idempotent),
schemas + deps wiring. ffmpeg already in the image.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/api/deps.py

@@ -24,6 +24,7 @@ from app.application.remote_library_service import RemoteLibraryService
 from app.application.streaming_service import StreamingService
 from app.application.subsonic_auth_service import SubsonicAuthService
 from app.application.sync_service import SyncService
+from app.application.transcode_service import TranscodeService
 from app.application.upload_service import UploadService
 from app.application.user_service import UserService
 from app.application.user_settings_service import UserSettingsService
@@ -178,6 +179,15 @@ def get_metadata_service(session: SessionDep, storage: FileStorageDep) -> Metada
     )
 
 
+def get_transcode_service(session: SessionDep) -> TranscodeService:
+    """Request-side cache lookups for transcoded renditions (§6.6). Generation
+    itself runs in the ``transcode_track`` worker, never here."""
+    return TranscodeService(
+        tracks=SqlAlchemyTrackRepository(session),
+        cache_root=get_settings().transcode_cache_path,
+    )
+
+
 def get_lyrics_service(session: SessionDep) -> LyricsService:
     """Wires the LRCLIB lyrics provider + cache repo (plan §6.7). LRCLIB is
     keyless, so this is always available; failures degrade to ``not_found``."""
@@ -215,6 +225,7 @@ UploadServiceDep = Annotated[UploadService, Depends(get_upload_service)]
 StreamingServiceDep = Annotated[StreamingService, Depends(get_streaming_service)]
 MetadataServiceDep = Annotated[MetadataEnrichmentService, Depends(get_metadata_service)]
 LyricsServiceDep = Annotated[LyricsService, Depends(get_lyrics_service)]
+TranscodeServiceDep = Annotated[TranscodeService, Depends(get_transcode_service)]
 DownloadServiceDep = Annotated[DownloadService, Depends(get_download_service)]
 RemoteLibraryServiceDep = Annotated[RemoteLibraryService, Depends(get_remote_library_service)]