feat(metadata): implement single-track metadata editor API (§A7/§1H)

Adds inline AcoustID match-finding (multiple ranked candidates via
lookup_all) and PUT /tracks/{id}/metadata for manual edits, resolving
artist/album and setting metadata_status=manual. Extends TrackOut with
genre/year/track_number.

tests/test_real_file_enrichment.py

@@ -145,6 +145,7 @@ def _pending_track() -> Track:
         duration_seconds=None,
         genre=None,
         year=None,
+        track_number=None,
         metadata_status="pending",
         metadata_error=None,
         enriched_at=None,