feat: cover-art pipeline (§1D)

Resolve, store and serve album cover art.

Sources (tag-first, mirroring enrichment): embedded artwork extracted
offline via mutagen (ID3 APIC / FLAC+OGG Picture / MP4 covr), then Cover
Art Archive by release-group MBID as a network fallback. Resolution runs
inside MetadataEnrichmentService after album resolution, only when the
album has no cover yet (idempotent, never overwrites), and is best-effort
so a cover failure never affects enrichment status.

- CoverArt value object + CoverArtExtractor/CoverArtProvider ports
- MutagenCoverExtractor + CoverArtArchiveClient adapters
- AcoustID parser now captures release_group_mbid
- Covers stored via FileStorage at covers/{album_id}.{ext} (local + S3)
- AlbumRepository.set_cover_path
- Serve real covers: GET /api/v1/albums|tracks/{id}/cover (StreamUser,
  ?token=), Subsonic getCoverArt (placeholder fallback)
- has_cover flag on AlbumOut/TrackOut
- coverart_enabled / coverart_base_url settings
- tests: cover resolution units + release_group parse + DB-backed
  test_cover_api.py (139 green via make test-api)

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

app/api/covers.py

@@ -0,0 +1,57 @@
+"""Shared cover-art serving helper (presentation).
+
+Streams a stored cover image from the :class:`FileStorage` port. Used by the
+native ``/api/v1`` cover endpoints and the Subsonic ``getCoverArt`` adapter so
+the streaming/content-type logic lives in one place.
+"""
+
+import uuid
+
+from fastapi.responses import StreamingResponse
+
+from app.domain.entities.album import Album
+from app.domain.errors import NotFoundError, StorageError
+from app.domain.ports import AlbumRepository, FileStorage, TrackRepository
+
+_CONTENT_TYPE_BY_EXT: dict[str, str] = {
+    "jpg": "image/jpeg",
+    "jpeg": "image/jpeg",
+    "png": "image/png",
+    "webp": "image/webp",
+    "gif": "image/gif",
+}
+
+# Covers are immutable for a given album (a new cover means a new key), so let
+# clients cache aggressively.
+_CACHE_CONTROL = "public, max-age=86400"
+
+
+def _content_type_for(key: str) -> str:
+    ext = key.rsplit(".", 1)[-1].lower() if "." in key else ""
+    return _CONTENT_TYPE_BY_EXT.get(ext, "application/octet-stream")
+
+
+async def stream_cover(storage: FileStorage, cover_path: str) -> StreamingResponse:
+    """Stream a stored cover by its storage key. Raises ``NotFoundError`` if the
+    object is missing (a dangling ``cover_path`` reads as "no cover")."""
+    try:
+        stream, total = await storage.open_range(cover_path, 0, None)
+    except StorageError as exc:
+        raise NotFoundError("Cover not found.") from exc
+    return StreamingResponse(
+        stream,
+        media_type=_content_type_for(cover_path),
+        headers={"Content-Length": str(total), "Cache-Control": _CACHE_CONTROL},
+    )
+
+
+async def resolve_album_for_track(
+    track_repo: TrackRepository,
+    album_repo: AlbumRepository,
+    track_id: uuid.UUID,
+) -> Album | None:
+    """The album that owns a track (cover lives on the album), or ``None``."""
+    track = await track_repo.get_by_id(track_id)
+    if track is None or track.album_id is None:
+        return None
+    return await album_repo.get_by_id(track.album_id)

app/api/rest/media.py

@@ -13,9 +13,17 @@ from typing import Annotated
 from fastapi import APIRouter, Header, Query
 from fastapi.responses import Response, StreamingResponse
 
-from app.api.deps import StreamingServiceDep, SubsonicUser, TrackRepoDep
-from app.api.rest.ids import decode_track, parse
-from app.domain.errors import NotFoundError
+from app.api.covers import resolve_album_for_track, stream_cover
+from app.api.deps import (
+    AlbumRepoDep,
+    FileStorageDep,
+    StreamingServiceDep,
+    SubsonicUser,
+    TrackRepoDep,
+)
+from app.api.rest.ids import IdKind, decode_track, parse
+from app.domain.entities.album import Album
+from app.domain.errors import NotFoundError, StorageError
 
 router = APIRouter()
 
@@ -69,10 +77,28 @@ async def download(
 @router.api_route("/getCoverArt.view", methods=["GET", "POST"])
 async def get_cover_art(
     _user: SubsonicUser,
+    album_repo: AlbumRepoDep,
+    track_repo: TrackRepoDep,
+    storage: FileStorageDep,
     id: Annotated[str, Query()],
     size: Annotated[int | None, Query()] = None,
 ) -> Response:
-    # Validate the id shape so clients get a clean error on garbage, then serve a
-    # placeholder. TODO: stream real covers once the cover pipeline exists.
-    parse(id)
+    # Cover ids reuse the entity id: ``al-<uuid>`` (album) or ``tr-<uuid>``
+    # (track → its album). Unlike the native API, Subsonic clients expect an
+    # image either way, so a missing cover falls back to a placeholder rather
+    # than 404. ``size`` is accepted but ignored (we serve the stored image).
+    kind, value = parse(id)
+    album: Album | None
+    if kind is IdKind.ALBUM:
+        album = await album_repo.get_by_id(value)
+    elif kind is IdKind.TRACK:
+        album = await resolve_album_for_track(track_repo, album_repo, value)
+    else:
+        album = None
+
+    if album is not None and album.cover_path:
+        try:
+            return await stream_cover(storage, album.cover_path)
+        except NotFoundError, StorageError:
+            pass
     return Response(content=_PLACEHOLDER_PNG, media_type="image/png")

app/api/schemas/album.py

@@ -13,4 +13,5 @@ class AlbumOut(BaseModel):
     artist_name: str
     year: int | None
     track_count: int
+    has_cover: bool
     created_at: dt.datetime

app/api/schemas/track.py

@@ -18,6 +18,7 @@ class TrackOut(BaseModel):
     file_size: int
     metadata_status: str
     source: str
+    has_cover: bool
     created_at: dt.datetime
 
 

app/api/v1/albums.py

@@ -1,11 +1,19 @@
 """Album endpoints."""
 
 import uuid
-from typing import Any
 
 from fastapi import APIRouter, Query
+from fastapi.responses import StreamingResponse
 
-from app.api.deps import AlbumRepoDep, ArtistRepoDep, CurrentUser, TrackRepoDep
+from app.api.covers import stream_cover
+from app.api.deps import (
+    AlbumRepoDep,
+    ArtistRepoDep,
+    CurrentUser,
+    FileStorageDep,
+    StreamUser,
+    TrackRepoDep,
+)
 from app.api.schemas.album import AlbumOut
 from app.api.schemas.pagination import PagedResponse
 from app.api.schemas.track import TrackOut
@@ -30,6 +38,7 @@ async def _build_album_out(
             artist_name=artists[a.artist_id].name if a.artist_id in artists else "Unknown Artist",
             year=a.year,
             track_count=track_counts.get(a.id, 0),
+            has_cover=bool(a.cover_path),
             created_at=a.created_at,
         )
         for a in albums
@@ -109,4 +118,14 @@ async def get_album_tracks(
 
 
 @router.get("/{album_id}/cover")
-async def get_album_cover(album_id: uuid.UUID, _: CurrentUser) -> Any: ...
+async def get_album_cover(
+    album_id: uuid.UUID,
+    album_repo: AlbumRepoDep,
+    storage: FileStorageDep,
+    _: StreamUser,
+) -> StreamingResponse:
+    # ``<img>`` can't send a bearer header → StreamUser accepts ``?token=``.
+    album = await album_repo.get_by_id(album_id)
+    if album is None or not album.cover_path:
+        raise NotFoundError("Cover not found.")
+    return await stream_cover(storage, album.cover_path)

app/api/v1/tracks.py

@@ -4,8 +4,17 @@ import uuid
 from typing import Any
 
 from fastapi import APIRouter, Query, Response
+from fastapi.responses import StreamingResponse
 
-from app.api.deps import AlbumRepoDep, ArtistRepoDep, CurrentUser, FileStorageDep, TrackRepoDep
+from app.api.covers import resolve_album_for_track, stream_cover
+from app.api.deps import (
+    AlbumRepoDep,
+    ArtistRepoDep,
+    CurrentUser,
+    FileStorageDep,
+    StreamUser,
+    TrackRepoDep,
+)
 from app.api.schemas.pagination import PagedResponse
 from app.api.schemas.track import TrackOut, TrackUpdate
 from app.domain.entities.album import Album
@@ -34,6 +43,7 @@ async def _build_track_out(
             file_size=t.file_size,
             metadata_status=t.metadata_status,
             source=t.source,
+            has_cover=bool(t.album_id and albums.get(t.album_id) and albums[t.album_id].cover_path),
             created_at=t.created_at,
         )
         for t in tracks
@@ -144,7 +154,19 @@ async def optimize_track(track_id: uuid.UUID, _: CurrentUser) -> Any: ...
 
 
 @router.get("/{track_id}/cover")
-async def get_track_cover(track_id: uuid.UUID, _: CurrentUser) -> Any: ...
+async def get_track_cover(
+    track_id: uuid.UUID,
+    track_repo: TrackRepoDep,
+    album_repo: AlbumRepoDep,
+    storage: FileStorageDep,
+    _: StreamUser,
+) -> StreamingResponse:
+    # A track's cover is its album's cover. ``<img>`` can't send a bearer
+    # header → StreamUser accepts ``?token=``.
+    album = await resolve_album_for_track(track_repo, album_repo, track_id)
+    if album is None or not album.cover_path:
+        raise NotFoundError("Cover not found.")
+    return await stream_cover(storage, album.cover_path)
 
 
 @router.post("/{track_id}/metadata/enrich")