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/application/lyrics_service.py

@@ -0,0 +1,97 @@
+"""Lyrics service (plan §6.7).
+
+Get-or-fetch with caching: a track's lyrics are served from the DB when present;
+on a miss (or an expired ``not_found``) we ask the provider (LRCLIB) once, then
+cache the outcome. ``not_found`` is cached with a TTL so tracks that genuinely
+have no lyrics aren't looked up on every play, but can eventually be retried.
+
+Degrades gracefully: if the provider is unreachable the lookup just yields a
+``not_found`` — the endpoint still returns 200 with empty lyrics, never an error.
+"""
+
+import datetime as dt
+import uuid
+
+from app.domain.entities.lyrics import Lyrics
+from app.domain.errors import NotFoundError
+from app.domain.ports import (
+    AlbumRepository,
+    ArtistRepository,
+    LyricsProvider,
+    LyricsRepository,
+    TrackRepository,
+)
+
+# Re-lookup a cached "not_found" only after this long — long enough not to spam
+# the provider, short enough that lyrics added upstream eventually surface.
+_NOT_FOUND_TTL = dt.timedelta(days=7)
+
+_STATUS_FOUND = "found"
+_STATUS_NOT_FOUND = "not_found"
+
+
+class LyricsService:
+    def __init__(
+        self,
+        *,
+        lyrics: LyricsRepository,
+        tracks: TrackRepository,
+        artists: ArtistRepository,
+        albums: AlbumRepository,
+        provider: LyricsProvider,
+    ) -> None:
+        self._lyrics = lyrics
+        self._tracks = tracks
+        self._artists = artists
+        self._albums = albums
+        self._provider = provider
+
+    async def get_lyrics(self, track_id: uuid.UUID, *, force: bool = False) -> Lyrics:
+        """Return cached lyrics, fetching from the provider on a miss/expiry.
+        ``force`` (the refetch endpoint) bypasses the cache entirely."""
+        cached = await self._lyrics.get(track_id)
+        if not force and cached is not None and self._is_fresh(cached):
+            return cached
+        return await self._fetch_and_cache(track_id)
+
+    def _is_fresh(self, cached: Lyrics) -> bool:
+        if cached.status == _STATUS_FOUND:
+            return True
+        if cached.status == _STATUS_NOT_FOUND:
+            return dt.datetime.now(dt.UTC) - cached.fetched_at < _NOT_FOUND_TTL
+        # "pending" (never fetched) → not fresh, go fetch.
+        return False
+
+    async def _fetch_and_cache(self, track_id: uuid.UUID) -> Lyrics:
+        track = await self._tracks.get_by_id(track_id)
+        if track is None:
+            raise NotFoundError(f"Track {track_id} not found.")
+
+        artist = await self._artists.get_by_id(track.artist_id)
+        album = (
+            await self._albums.get_by_id(track.album_id)
+            if track.album_id is not None
+            else None
+        )
+        result = await self._provider.fetch(
+            artist=artist.name if artist else "",
+            title=track.title,
+            album=album.title if album else None,
+            duration_seconds=track.duration_seconds,
+        )
+
+        if result is None:
+            return await self._lyrics.upsert(
+                track_id=track_id,
+                synced=None,
+                plain=None,
+                source=None,
+                status=_STATUS_NOT_FOUND,
+            )
+        return await self._lyrics.upsert(
+            track_id=track_id,
+            synced=result.synced,
+            plain=result.plain,
+            source=result.source,
+            status=_STATUS_FOUND,
+        )