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.

app/domain/entities/track.py

@@ -27,6 +27,7 @@ class Track:
     duration_seconds: int | None
     genre: str | None
     year: int | None
+    track_number: int | None
     metadata_status: str
     metadata_error: str | None
     enriched_at: dt.datetime | None

app/domain/ports.py

@@ -302,10 +302,13 @@ class AudioFingerprinter(Protocol):
 class AcoustIdClient(Protocol):
     """AcoustID lookup. ``is_available`` is False without an API key (the whole
     fingerprint path is then skipped). ``lookup`` returns the best match or
-    ``None`` (no result / service down), never raising."""
+    ``None`` (no result / service down), never raising. ``lookup_all`` returns
+    the same candidates ranked by confidence (``[]`` on no result / unavailable
+    / error), for the metadata editor's match picker."""
 
     def is_available(self) -> bool: ...
     async def lookup(self, fingerprint: Fingerprint) -> RecordingMatch | None: ...
+    async def lookup_all(self, fingerprint: Fingerprint) -> list[RecordingMatch]: ...
 
 
 class CoverArtExtractor(Protocol):