feat(subsonic): getLyricsBySongId over the native LyricsService

Adapts the cached LyricsService into the OpenSubsonic structured-lyrics shape:
synced LRC parsed into timed lines (ms), plain text as untimed fallback, empty
lyricsList when a track has none. Thin adapter — no logic duplicated. Also
parenthesize the getCoverArt except-tuple for clarity.

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

app/api/rest/media.py

@@ -1,10 +1,11 @@
-"""Subsonic media endpoints: stream, download, cover art.
+"""Subsonic media endpoints: stream, download, cover art, lyrics.
 
 ``stream`` and ``download`` reuse :class:`StreamingService` (honouring HTTP
 Range) — they return raw bytes, not the Subsonic envelope. Transcoding params
 (``maxBitRate``/``format``) are accepted but ignored; the original file is served
-(no in-request ffmpeg — CLAUDE.md). ``getCoverArt`` returns a placeholder until
-the cover pipeline lands (the ``/api/v1`` cover endpoints are still stubs).
+(no in-request ffmpeg — CLAUDE.md). ``getCoverArt`` serves the album cover (a
+placeholder when there's none). ``getLyricsBySongId`` adapts the native
+``LyricsService`` into the OpenSubsonic structured-lyrics shape.
 """
 
 import base64
@@ -16,12 +17,17 @@ from fastapi.responses import Response, StreamingResponse
 from app.api.covers import resolve_album_for_track, stream_cover
 from app.api.deps import (
     AlbumRepoDep,
+    ArtistRepoDep,
     FileStorageDep,
+    LyricsServiceDep,
     StreamingServiceDep,
+    SubsonicFormat,
     SubsonicUser,
     TrackRepoDep,
 )
+from app.api.rest.envelope import subsonic_response
 from app.api.rest.ids import IdKind, decode_track, parse
+from app.api.rest.serializers import structured_lyrics
 from app.domain.entities.album import Album
 from app.domain.errors import NotFoundError, StorageError
 
@@ -99,6 +105,35 @@ async def get_cover_art(
     if album is not None and album.cover_path:
         try:
             return await stream_cover(storage, album.cover_path)
-        except NotFoundError, StorageError:
+        except (NotFoundError, StorageError):
             pass
     return Response(content=_PLACEHOLDER_PNG, media_type="image/png")
+
+
+@router.api_route("/getLyricsBySongId", methods=["GET", "POST"])
+@router.api_route("/getLyricsBySongId.view", methods=["GET", "POST"])
+async def get_lyrics_by_song_id(
+    _user: SubsonicUser,
+    fmt: SubsonicFormat,
+    lyrics_service: LyricsServiceDep,
+    track_repo: TrackRepoDep,
+    artist_repo: ArtistRepoDep,
+    id: Annotated[str, Query()],
+) -> Response:
+    # OpenSubsonic structured lyrics over the native LyricsService (§6.7). A miss
+    # is a normal empty ``lyricsList`` — the service degrades to not_found rather
+    # than raising, so clients get 200 either way.
+    track_id = decode_track(id)
+    track = await track_repo.get_by_id(track_id)
+    if track is None:
+        raise NotFoundError("Song not found.")
+    lyrics = await lyrics_service.get_lyrics(track_id)
+    artist = await artist_repo.get_by_id(track.artist_id)
+    return subsonic_response(
+        structured_lyrics(
+            lyrics,
+            display_artist=artist.name if artist is not None else "",
+            display_title=track.title,
+        ),
+        fmt=fmt,
+    )