feat(library): lazy materialization foundation for remote tracks (§Phase1)

Adds nullable storage fields + availability column on tracks, remote
source/source_id identity on albums/artists, TrackRepository.materialize()
and get_or_create_remote() repos — groundwork for on-demand YTM library
(placeholders saved without audio, materialized in-place on first play).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/api/v1/tracks.py

@@ -54,6 +54,7 @@ async def _build_track_out(
             metadata_status=t.metadata_status,
             metadata_error=t.metadata_error,
             enriched_at=t.enriched_at,
+            availability=t.availability,
             source=t.source,
             has_cover=bool(t.album_id and albums.get(t.album_id) and albums[t.album_id].cover_path),
             created_at=t.created_at,
@@ -155,7 +156,8 @@ async def delete_track(
     if track is None:
         raise NotFoundError(f"Track {track_id} not found.")
     await track_repo.delete(track_id)
-    await storage.delete(track.storage_uri)
+    if track.storage_uri is not None:
+        await storage.delete(track.storage_uri)
     return Response(status_code=204)