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/__init__.py

@@ -7,6 +7,7 @@ from app.infrastructure.db.repositories.download_job_repository import (
 )
 from app.infrastructure.db.repositories.history_repository import SqlAlchemyHistoryRepository
 from app.infrastructure.db.repositories.like_repository import SqlAlchemyLikeRepository
+from app.infrastructure.db.repositories.lyrics_repository import SqlAlchemyLyricsRepository
 from app.infrastructure.db.repositories.playlist_repository import SqlAlchemyPlaylistRepository
 from app.infrastructure.db.repositories.refresh_token_repository import (
     SqlAlchemyRefreshTokenRepository,
@@ -23,6 +24,7 @@ __all__ = [
     "SqlAlchemyDownloadJobRepository",
     "SqlAlchemyHistoryRepository",
     "SqlAlchemyLikeRepository",
+    "SqlAlchemyLyricsRepository",
     "SqlAlchemyPlaylistRepository",
     "SqlAlchemyRefreshTokenRepository",
     "SqlAlchemyTrackRepository",