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/infrastructure/sources/registry.py

@@ -2,16 +2,18 @@
 
 Built from settings at the composition root. Only sources that are configured
 are registered (e.g. ``local`` appears only when ``LOCAL_MEDIA_IMPORT_PATH`` is
-set), so enumeration reflects what the instance can actually use.
+set; ``youtube`` only when ``YOUTUBE_ENABLED``), so enumeration reflects what the
+instance can actually use.
 """
 
 from typing import cast
 
 from app.core.config import Settings
 from app.domain.errors import NotFoundError, ValidationError
-from app.domain.ports import IndexableSource, SourceBackend
+from app.domain.ports import FetchableSource, IndexableSource, SearchableSource, SourceBackend
 from app.domain.sources import SourceInfo
 from app.infrastructure.sources.local_folder import LocalFolderSource
+from app.infrastructure.sources.youtube import YouTubeMusicSource
 
 
 class SourceRegistry:
@@ -30,6 +32,22 @@ class SourceRegistry:
             raise ValidationError(f"Source {name!r} cannot be indexed.")
         return cast(IndexableSource, backend)
 
+    def searchable(self, name: str) -> SearchableSource:
+        backend = self.get(name)
+        if not hasattr(backend, "search"):
+            raise ValidationError(f"Source {name!r} cannot be searched.")
+        return cast(SearchableSource, backend)
+
+    def fetchable(self, name: str) -> FetchableSource:
+        backend = self.get(name)
+        if not hasattr(backend, "fetch"):
+            raise ValidationError(f"Source {name!r} cannot download.")
+        return cast(FetchableSource, backend)
+
+    def searchables(self) -> list[SearchableSource]:
+        """Every registered source that supports search (for cross-source search)."""
+        return [cast(SearchableSource, b) for b in self._by_name.values() if hasattr(b, "search")]
+
     def infos(self) -> list[SourceInfo]:
         return [backend.info() for backend in self._by_name.values()]
 
@@ -38,4 +56,11 @@ def build_source_registry(settings: Settings) -> SourceRegistry:
     backends: list[SourceBackend] = []
     if settings.local_media_import_path is not None:
         backends.append(LocalFolderSource(settings.local_media_import_path))
+    if settings.youtube_enabled:
+        backends.append(
+            YouTubeMusicSource(
+                cookies_path=settings.youtube_cookies_path,
+                tmp_dir=settings.upload_tmp_dir,
+            )
+        )
     return SourceRegistry(backends)