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>

tests/test_metadata_api.py

@@ -107,9 +107,7 @@ async def test_track_out_includes_genre_year_track_number(api: AsyncClient) -> N
     token = await _login(api)
     track_id = await _upload(api, token)
 
-    resp = await api.get(
-        f"/api/v1/tracks/{track_id}", headers={"Authorization": f"Bearer {token}"}
-    )
+    resp = await api.get(f"/api/v1/tracks/{track_id}", headers={"Authorization": f"Bearer {token}"})
     assert resp.status_code == 200, resp.text
     body = resp.json()
     assert "genre" in body