feat(sources): YouTube Music search + download pipeline (§1C/§1E)

Pluggable fetch source: ytmusicapi search + yt-dlp download (cookies-file guard), DownloadJob entity/repo + DownloadService, download_task worker with exponential-backoff retries, and wired /search, /sources/{source}/search, and /downloads endpoints. Adds youtube_enabled/cookies config, yt-dlp+ytmusicapi deps, and the download_jobs.track_id migration. Snapshot also bundles in-progress storage/tracks/acoustid edits.

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

app/api/deps.py

@@ -15,6 +15,7 @@ from fastapi.security import HTTPAuthorizationCredentials, HTTPBearer
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.application.auth_service import AuthService
+from app.application.download_service import DownloadService
 from app.application.metadata_service import MetadataEnrichmentService
 from app.application.streaming_service import StreamingService
 from app.application.subsonic_auth_service import SubsonicAuthService
@@ -29,6 +30,7 @@ from app.infrastructure.db import get_sessionmaker
 from app.infrastructure.db.repositories import (
     SqlAlchemyAlbumRepository,
     SqlAlchemyArtistRepository,
+    SqlAlchemyDownloadJobRepository,
     SqlAlchemyHistoryRepository,
     SqlAlchemyLikeRepository,
     SqlAlchemyPlaylistRepository,
@@ -41,7 +43,7 @@ from app.infrastructure.metadata.fingerprint import FpcalcFingerprinter
 from app.infrastructure.metadata.tags import MutagenTagReader
 from app.infrastructure.sources.registry import SourceRegistry, build_source_registry
 from app.infrastructure.storage.provider import get_file_storage
-from app.workers.queue import enqueue_enrich
+from app.workers.queue import enqueue_download, enqueue_enrich
 
 
 async def get_session() -> AsyncIterator[AsyncSession]:
@@ -136,9 +138,7 @@ def get_streaming_service(session: SessionDep, storage: FileStorageDep) -> Strea
     )
 
 
-def get_metadata_service(
-    session: SessionDep, storage: FileStorageDep
-) -> MetadataEnrichmentService:
+def get_metadata_service(session: SessionDep, storage: FileStorageDep) -> MetadataEnrichmentService:
     """Wires the §6.2 fingerprint/AcoustID adapters for read-only, inline use
     (the metadata editor's "find matches" — §A7). The full pipeline (incl.
     cover art) stays in the worker (`tasks/enrich_task.py`)."""
@@ -161,9 +161,21 @@ def get_metadata_service(
     )
 
 
+def get_download_service(session: SessionDep, storage: FileStorageDep) -> DownloadService:
+    return DownloadService(
+        jobs=SqlAlchemyDownloadJobRepository(session),
+        tracks=SqlAlchemyTrackRepository(session),
+        artists=SqlAlchemyArtistRepository(session),
+        storage=storage,
+        enqueue_download=enqueue_download,
+        enqueue_enrich=enqueue_enrich,
+    )
+
+
 UploadServiceDep = Annotated[UploadService, Depends(get_upload_service)]
 StreamingServiceDep = Annotated[StreamingService, Depends(get_streaming_service)]
 MetadataServiceDep = Annotated[MetadataEnrichmentService, Depends(get_metadata_service)]
+DownloadServiceDep = Annotated[DownloadService, Depends(get_download_service)]
 
 
 # -- library repository deps ---------------------------------------------------