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/api/schemas/lyrics.py

@@ -0,0 +1,33 @@
+"""Lyrics response schema (§6.7 / Now Playing lyrics panel).
+
+Returns the raw LRC (``synced``) and/or ``plain`` text; the client parses LRC
+timestamps for synced highlighting. A miss is a normal 200 with
+``status="not_found"`` and null text — not an error — so the panel can render a
+"no lyrics" state.
+"""
+
+import uuid
+
+from pydantic import BaseModel
+
+from app.domain.entities.lyrics import Lyrics
+
+
+class LyricsOut(BaseModel):
+    track_id: uuid.UUID
+    status: str
+    source: str | None
+    synced: str | None
+    plain: str | None
+    synced_available: bool
+
+    @classmethod
+    def from_entity(cls, lyrics: Lyrics) -> LyricsOut:
+        return cls(
+            track_id=lyrics.track_id,
+            status=lyrics.status,
+            source=lyrics.source,
+            synced=lyrics.synced,
+            plain=lyrics.plain,
+            synced_available=lyrics.synced is not None,
+        )