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/v1/search.py

@@ -1,12 +1,11 @@
 """Search endpoints: global and library-scoped."""
 
-from typing import Any
-
 from fastapi import APIRouter, Query
 
-from app.api.deps import AlbumRepoDep, ArtistRepoDep, CurrentUser, TrackRepoDep
+from app.api.deps import AlbumRepoDep, ArtistRepoDep, CurrentUser, SourceRegistryDep, TrackRepoDep
 from app.api.schemas.album import AlbumOut
 from app.api.schemas.artist import ArtistOut
+from app.api.schemas.external_search import ExternalSearchResponse, ExternalSearchResultOut
 from app.api.schemas.search import LibrarySearchResponse
 from app.api.schemas.track import TrackOut
 from app.api.v1.albums import _build_album_out
@@ -16,7 +15,26 @@ router = APIRouter(prefix="/search", tags=["search"])
 
 
 @router.get("")
-async def search(_: CurrentUser) -> Any: ...
+async def search(
+    _: CurrentUser,
+    registry: SourceRegistryDep,
+    q: str = Query(min_length=1),
+    limit: int = Query(20, ge=1, le=50),
+) -> ExternalSearchResponse:
+    """Search every available fetch source and merge the hits (§A4 discover).
+
+    A source that is down contributes nothing rather than failing the whole
+    request (graceful degradation); only available sources are reported as
+    searched."""
+    results: list[ExternalSearchResultOut] = []
+    searched: list[str] = []
+    for backend in registry.searchables():
+        if not backend.is_available():
+            continue
+        searched.append(backend.name)
+        hits = await backend.search(q, limit=limit)
+        results.extend(ExternalSearchResultOut.from_entity(h) for h in hits)
+    return ExternalSearchResponse(results=results, searched_sources=searched)
 
 
 @router.get("/library")