feat(lyrics): LRCLIB provider + cached lyrics endpoints (§6.7)

GET /tracks/{id}/lyrics (get-or-fetch, caches found/not_found with a 7-day
miss TTL) and POST /tracks/{id}/lyrics/refetch (force). Hexagonal wiring:
LyricsProvider/LyricsRepository ports, LrclibHttpClient adapter (keyless,
degrades to not_found on error), SqlAlchemyLyricsRepository (upsert on the
existing lyrics table), LyricsService, LyricsOut schema, deps.py factory.

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

app/infrastructure/db/repositories/lyrics_repository.py

@@ -0,0 +1,72 @@
+"""Lyrics repository — adapter over ``AsyncSession``.
+
+One cached row per track (``track_id`` unique). ``upsert`` refreshes the row and
+bumps ``fetched_at`` so the service's TTL is measured from the last fetch.
+"""
+
+import uuid
+
+from sqlalchemy import func, select
+from sqlalchemy.dialects.postgresql import insert as pg_insert
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.domain.entities.lyrics import Lyrics
+from app.infrastructure.db.models.lyrics import LyricsModel
+
+
+def _to_entity(row: LyricsModel) -> Lyrics:
+    return Lyrics(
+        track_id=row.track_id,
+        synced=row.synced,
+        plain=row.plain,
+        source=row.source,
+        status=row.status,
+        fetched_at=row.fetched_at,
+    )
+
+
+class SqlAlchemyLyricsRepository:
+    def __init__(self, session: AsyncSession) -> None:
+        self._session = session
+
+    async def get(self, track_id: uuid.UUID) -> Lyrics | None:
+        row = await self._session.scalar(
+            select(LyricsModel).where(LyricsModel.track_id == track_id)
+        )
+        return _to_entity(row) if row is not None else None
+
+    async def upsert(
+        self,
+        *,
+        track_id: uuid.UUID,
+        synced: str | None,
+        plain: str | None,
+        source: str | None,
+        status: str,
+    ) -> Lyrics:
+        values = {
+            "track_id": track_id,
+            "synced": synced,
+            "plain": plain,
+            "source": source,
+            "status": status,
+            "fetched_at": func.now(),
+        }
+        stmt = (
+            pg_insert(LyricsModel)
+            .values(**values)
+            .on_conflict_do_update(
+                index_elements=[LyricsModel.track_id],
+                set_={
+                    "synced": synced,
+                    "plain": plain,
+                    "source": source,
+                    "status": status,
+                    "fetched_at": func.now(),
+                },
+            )
+            .returning(LyricsModel)
+        )
+        row = (await self._session.scalars(stmt)).one()
+        await self._session.flush()
+        return _to_entity(row)