feat(reco): radio + similar with metadata fallback (§6.5)

POST /radio + /radio/next (stateless infinite feed: seed track / from-likes,
exploration mix, client-passed exclude_ids) and GET /tracks|artists/{id}/similar,
replacing the stubs. Recommender port abstracts the (future) ML service —
NullRecommender is wired now so RecommendationService always uses its metadata
heuristics (genre/artist similarity, random exploration filler), never a hard ML
dependency. Adds TrackRepository.list_similar/sample_playable + Artist.list_similar,
reason codes for the client, RemoteRecommender skeleton (TODO: ML contract).

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

app/infrastructure/db/repositories/artist_repository.py

@@ -77,6 +77,26 @@ class SqlAlchemyArtistRepository:
         )
         return [_to_entity(r) for r in rows]
 
+    async def list_similar(self, *, artist_id: uuid.UUID, limit: int) -> list[Artist]:
+        # Artists whose tracks fall in the seed artist's genres, ranked by how
+        # many such tracks they have. Defined before ``list`` so the ``list[Artist]``
+        # return annotation isn't shadowed by the method named ``list``.
+        seed_genres = (
+            select(TrackModel.genre)
+            .where(TrackModel.artist_id == artist_id, TrackModel.genre.is_not(None))
+            .distinct()
+        )
+        stmt = (
+            select(ArtistModel)
+            .join(TrackModel, TrackModel.artist_id == ArtistModel.id)
+            .where(TrackModel.genre.in_(seed_genres), ArtistModel.id != artist_id)
+            .group_by(ArtistModel.id)
+            .order_by(func.count(TrackModel.id).desc())
+            .limit(limit)
+        )
+        rows = (await self._session.execute(stmt)).scalars().all()
+        return [_to_entity(r) for r in rows]
+
     async def list(self, *, q: str | None, limit: int, offset: int) -> list[Artist]:
         stmt = select(ArtistModel)
         if q:
@@ -108,3 +128,4 @@ class SqlAlchemyArtistRepository:
                 .where(TrackModel.artist_id == artist_id)
             )
         ).scalar_one()
+

app/infrastructure/db/repositories/track_repository.py

@@ -3,7 +3,7 @@
 import datetime as dt
 import uuid
 
-from sqlalchemy import func, select
+from sqlalchemy import case, func, or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.domain.entities.storage import FormatBreakdown, LibraryStats
@@ -136,6 +136,42 @@ class SqlAlchemyTrackRepository:
         ).all()
         return [(row.genre, row.cnt) for row in rows]
 
+    async def list_similar(
+        self,
+        *,
+        genre: str | None,
+        artist_id: uuid.UUID,
+        exclude_ids: list[uuid.UUID],
+        limit: int,
+    ) -> list[Track]:
+        # Rank a same-genre hit above a same-artist hit; shuffle within a tier so
+        # the mix varies. Only playable (locally-stored) tracks are candidates.
+        if genre is not None:
+            match = or_(TrackModel.genre == genre, TrackModel.artist_id == artist_id)
+            score = case((TrackModel.genre == genre, 2), else_=0) + case(
+                (TrackModel.artist_id == artist_id, 1), else_=0
+            )
+        else:
+            match = TrackModel.artist_id == artist_id
+            score = case((TrackModel.artist_id == artist_id, 1), else_=0)
+
+        stmt = select(TrackModel).where(TrackModel.storage_uri.is_not(None), match)
+        if exclude_ids:
+            stmt = stmt.where(TrackModel.id.not_in(exclude_ids))
+        stmt = stmt.order_by(score.desc(), func.random()).limit(limit)
+        rows = (await self._session.execute(stmt)).scalars().all()
+        return [_to_entity(r) for r in rows]
+
+    async def sample_playable(
+        self, *, exclude_ids: list[uuid.UUID], limit: int
+    ) -> list[Track]:
+        stmt = select(TrackModel).where(TrackModel.storage_uri.is_not(None))
+        if exclude_ids:
+            stmt = stmt.where(TrackModel.id.not_in(exclude_ids))
+        stmt = stmt.order_by(func.random()).limit(limit)
+        rows = (await self._session.execute(stmt)).scalars().all()
+        return [_to_entity(r) for r in rows]
+
     async def library_stats(self) -> LibraryStats:
         """One-shot aggregate over the whole catalogue (no pagination). Defined
         before ``list`` for the same shadowing reason as ``genres``."""