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>

pyproject.toml

@@ -27,6 +27,9 @@ dependencies = [
     "httpx>=0.28",
     # embedded audio tag reading (enrichment tag pre-pass)
     "mutagen>=1.47",
+    # youtube source: search (ytmusicapi) + download (yt-dlp)
+    "ytmusicapi>=1.8",
+    "yt-dlp>=2024.12.13",
     # S3-compatible object storage
     "aioboto3>=13.0",
     # logging
@@ -95,6 +98,10 @@ ignore_missing_imports = true
 module = ["aioboto3.*", "aiobotocore.*", "botocore.*"]
 ignore_missing_imports = true
 
+[[tool.mypy.overrides]]
+module = ["ytmusicapi.*", "yt_dlp.*"]
+ignore_missing_imports = true
+
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
 testpaths = ["tests"]