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/api/schemas/track.py

@@ -16,6 +16,9 @@ class TrackOut(BaseModel):
     duration_seconds: int | None
     file_format: str
     file_size: int
+    genre: str | None
+    year: int | None
+    track_number: int | None
     metadata_status: str
     metadata_error: str | None
     enriched_at: dt.datetime | None
@@ -28,3 +31,33 @@ class TrackUpdate(BaseModel):
     title: str | None = None
     genre: str | None = None
     year: int | None = None
+
+
+class MetadataMatch(BaseModel):
+    """One AcoustID candidate for the metadata editor's match picker (§A7)."""
+
+    acoustid: str
+    score: float
+    recording_mbid: str | None
+    release_group_mbid: str | None
+    title: str | None
+    artist: str | None
+    album: str | None
+    year: int | None
+
+
+class MetadataMatchesOut(BaseModel):
+    items: list[MetadataMatch]
+
+
+class MetadataApply(BaseModel):
+    """Manual edits / accepted match applied via ``PUT /tracks/{id}/metadata``.
+
+    Sets ``metadata_status = manual`` (never overwritten by auto-enrichment)."""
+
+    title: str | None = None
+    artist_name: str | None = None
+    album_title: str | None = None
+    year: int | None = None
+    genre: str | None = None
+    track_number: int | None = None