feat(tracks): filter track list by ingest source

Add an optional `source` filter to `GET /api/v1/tracks` (and the
`TrackRepository.list`/`count` port + SQLAlchemy adapter). Lets clients
query, e.g., only uploaded tracks (`?source=upload`) newest-first — the
backing for the webui's persistent "Recently uploaded" view.

- test: upload then list with `?source=upload` (hit) / `?source=youtube`
  (miss)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/domain/ports.py

@@ -145,6 +145,7 @@ class TrackRepository(Protocol):
         artist_id: uuid.UUID | None,
         album_id: uuid.UUID | None,
         q: str | None,
+        source: str | None = None,
         sort_by: str,
         order: str,
         limit: int,
@@ -156,6 +157,7 @@ class TrackRepository(Protocol):
         artist_id: uuid.UUID | None,
         album_id: uuid.UUID | None,
         q: str | None,
+        source: str | None = None,
     ) -> int: ...
     async def update(
         self,