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_import_service.py

@@ -51,6 +51,7 @@ class FakeTrackRepo:
             duration_seconds=None,
             genre=None,
             year=None,
+            track_number=None,
             metadata_status=str(kw["metadata_status"]),
             metadata_error=None,
             enriched_at=None,
@@ -134,6 +135,7 @@ async def test_dedup_skips_already_imported() -> None:
         duration_seconds=None,
         genre=None,
         year=None,
+        track_number=None,
         metadata_status="pending",
         metadata_error=None,
         enriched_at=None,