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_metadata_service.py

@@ -38,6 +38,7 @@ def _track(*, metadata_status: str = "pending", title: str = "raw-stem") -> Trac
         duration_seconds=None,
         genre=None,
         year=None,
+        track_number=None,
         metadata_status=metadata_status,
         metadata_error=None,
         enriched_at=None,
@@ -165,6 +166,10 @@ class FakeAcoustId:
         self.calls += 1
         return self._match
 
+    async def lookup_all(self, fingerprint: Fingerprint) -> list[RecordingMatch]:
+        self.calls += 1
+        return [self._match] if self._match is not None else []
+
 
 def _service(
     *,