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/domain/entities/__init__.py

@@ -2,6 +2,7 @@
 
 from app.domain.entities.album import Album
 from app.domain.entities.cover import CoverArt
+from app.domain.entities.download import DownloadJob
 from app.domain.entities.history import PlayHistoryEntry
 from app.domain.entities.like import Like
 from app.domain.entities.metadata import AudioTags, Fingerprint, RecordingMatch
@@ -22,6 +23,7 @@ __all__ = [
     "CoverArt",
     "Credentials",
     "DiskUsage",
+    "DownloadJob",
     "Fingerprint",
     "FormatBreakdown",
     "LibraryStats",