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/workers/arq_worker.py

@@ -1,7 +1,6 @@
 """arq worker settings — the queue runtime. Task functions register here.
 
 Run with: ``arq app.workers.arq_worker.WorkerSettings``.
-Tasks (download, transcode) are appended to ``functions`` in later steps.
 """
 
 from typing import Any, ClassVar
@@ -10,6 +9,7 @@ from arq.connections import RedisSettings
 
 from app.core.config import get_settings
 from app.core.logging import configure_logging, get_logger
+from app.workers.tasks.download_task import download_track
 from app.workers.tasks.enrich_task import enrich_track
 from app.workers.tasks.import_task import scan_local_folder
 
@@ -27,7 +27,7 @@ async def shutdown(_ctx: dict[str, Any]) -> None:
 
 
 class WorkerSettings:
-    functions: ClassVar[list[Any]] = [scan_local_folder, enrich_track]
+    functions: ClassVar[list[Any]] = [scan_local_folder, enrich_track, download_track]
     on_startup = startup
     on_shutdown = shutdown
     max_jobs = get_settings().max_parallel_downloads