feat(enrichment): tag-first metadata pipeline (§1D)

Implements the §6.2 enrichment pipeline: embedded tags → Chromaprint
fingerprint → AcoustID lookup. Well-tagged files get correct
artist/album/title offline; the rest are identified via AcoustID
(which also yields a MusicBrainz recording id in one call).

- domain: AudioTags/Fingerprint/RecordingMatch value objects; ports
  AudioTagReader, AudioFingerprinter, AcoustIdClient; TrackRepository
  .apply_enrichment (gap-fill, never erases) + AlbumRepository.get_or_create
- infrastructure/metadata: MutagenTagReader, FpcalcFingerprinter,
  AcoustIdHttpClient (rich meta=recordings+releasegroups, throttled)
- application: MetadataEnrichmentService — tags preferred, AcoustID fills
  gaps; resolves artist/album; status enriched/failed; skips manual;
  every external step wrapped (graceful degradation)
- workers: enrich_task registered; enqueue_enrich is best-effort and
  deferred so the caller's txn commits before the worker reads the row
- wiring: upload enqueues after add; import returns imported_ids and
  enqueues post-commit (mid-scan would race the worker); manual
  POST /tracks/{id}/metadata/enrich endpoint
- deps: add mutagen (fpcalc/ffmpeg already in the image)

Tests: metadata service orchestration, AcoustID parser, tag helpers.
125 passed; mypy strict + ruff clean.

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

tests/test_metadata_service.py

@@ -0,0 +1,283 @@
+"""Unit tests for MetadataEnrichmentService — DB-free, in-memory fakes.
+
+Covers the §6.2 orchestration contract: tag-first merge, AcoustID fallback,
+artist/album resolution, status transitions, and the hard invariants
+(``manual`` untouched, graceful degradation, idempotent gap-fill).
+"""
+
+import datetime as dt
+import uuid
+from collections.abc import AsyncIterator
+from contextlib import asynccontextmanager
+from pathlib import Path
+
+import pytest
+from app.application.metadata_service import MetadataEnrichmentService
+from app.domain.entities import Artist, Track
+from app.domain.entities.album import Album
+from app.domain.entities.metadata import AudioTags, Fingerprint, RecordingMatch
+
+pytestmark = pytest.mark.asyncio
+
+_UNKNOWN = "Unknown Artist"
+
+
+def _track(*, metadata_status: str = "pending", title: str = "raw-stem") -> Track:
+    now = dt.datetime.now(dt.UTC)
+    return Track(
+        id=uuid.uuid4(),
+        title=title,
+        artist_id=uuid.uuid4(),  # the "Unknown Artist" id
+        album_id=None,
+        storage_uri="tracks/aa/song.mp3",
+        file_format="mp3",
+        file_size=123,
+        source="upload",
+        source_id="deadbeef",
+        duration_seconds=None,
+        genre=None,
+        year=None,
+        metadata_status=metadata_status,
+        created_at=now,
+        updated_at=now,
+    )
+
+
+class FakeTrackRepo:
+    def __init__(self, track: Track | None) -> None:
+        self._track = track
+        self.applied: dict[str, object] | None = None
+
+    async def get_by_id(self, track_id: uuid.UUID) -> Track | None:
+        return self._track
+
+    async def apply_enrichment(self, track_id: uuid.UUID, **kw: object) -> Track:
+        self.applied = kw
+        return self._track  # type: ignore[return-value]
+
+
+class FakeArtistRepo:
+    def __init__(self) -> None:
+        self.created: list[str] = []
+
+    async def get_or_create(self, name: str) -> Artist:
+        self.created.append(name)
+        now = dt.datetime.now(dt.UTC)
+        return Artist(id=uuid.uuid4(), name=name, created_at=now, updated_at=now)
+
+
+class FakeAlbumRepo:
+    def __init__(self) -> None:
+        self.created: list[tuple[str, uuid.UUID]] = []
+
+    async def get_or_create(
+        self, *, title: str, artist_id: uuid.UUID, year: int | None, musicbrainz_id: str | None
+    ) -> Album:
+        self.created.append((title, artist_id))
+        now = dt.datetime.now(dt.UTC)
+        return Album(
+            id=uuid.uuid4(),
+            title=title,
+            artist_id=artist_id,
+            year=year,
+            cover_path=None,
+            musicbrainz_id=musicbrainz_id,
+            created_at=now,
+            updated_at=now,
+        )
+
+
+class FakeStorage:
+    @asynccontextmanager
+    async def as_local_path(self, key: str) -> AsyncIterator[Path]:
+        yield Path("/tmp") / key
+
+
+class FakeTagReader:
+    def __init__(self, tags: AudioTags | None) -> None:
+        self._tags = tags
+
+    async def read(self, path: Path) -> AudioTags | None:
+        return self._tags
+
+
+class FakeFingerprinter:
+    def __init__(self, fp: Fingerprint | None, *, available: bool = True) -> None:
+        self._fp = fp
+        self._available = available
+
+    def is_available(self) -> bool:
+        return self._available
+
+    async def calculate(self, path: Path) -> Fingerprint | None:
+        return self._fp
+
+
+class FakeAcoustId:
+    def __init__(self, match: RecordingMatch | None, *, available: bool = True) -> None:
+        self._match = match
+        self._available = available
+        self.calls = 0
+
+    def is_available(self) -> bool:
+        return self._available
+
+    async def lookup(self, fingerprint: Fingerprint) -> RecordingMatch | None:
+        self.calls += 1
+        return self._match
+
+
+def _service(
+    *,
+    track: Track | None,
+    tags: AudioTags | None = None,
+    fp: Fingerprint | None = None,
+    match: RecordingMatch | None = None,
+    fp_available: bool = True,
+    acoustid_available: bool = True,
+) -> tuple[MetadataEnrichmentService, FakeTrackRepo, FakeArtistRepo, FakeAlbumRepo, FakeAcoustId]:
+    tracks = FakeTrackRepo(track)
+    artists = FakeArtistRepo()
+    albums = FakeAlbumRepo()
+    acoustid = FakeAcoustId(match, available=acoustid_available)
+    service = MetadataEnrichmentService(
+        tracks=tracks,  # type: ignore[arg-type]
+        artists=artists,  # type: ignore[arg-type]
+        albums=albums,  # type: ignore[arg-type]
+        storage=FakeStorage(),  # type: ignore[arg-type]
+        tag_reader=FakeTagReader(tags),  # type: ignore[arg-type]
+        fingerprinter=FakeFingerprinter(fp, available=fp_available),  # type: ignore[arg-type]
+        acoustid=acoustid,  # type: ignore[arg-type]
+    )
+    return service, tracks, artists, albums, acoustid
+
+
+async def test_tags_only_enriches_and_relinks_artist_and_album() -> None:
+    track = _track()
+    tags = AudioTags(
+        title="Real Title",
+        artist="Pink Floyd",
+        album="The Wall",
+        genre="Rock",
+        year=1979,
+        track_number=1,
+        duration_seconds=222,
+    )
+    service, tracks, artists, albums, acoustid = _service(track=track, tags=tags)
+
+    result = await service.enrich(track.id)
+
+    assert result.status == "enriched"
+    assert acoustid.calls == 0  # no fingerprint → no lookup needed
+    assert "Pink Floyd" in artists.created
+    assert albums.created and albums.created[0][0] == "The Wall"
+    applied = tracks.applied
+    assert applied is not None
+    assert applied["title"] == "Real Title"
+    assert applied["genre"] == "Rock"
+    assert applied["year"] == 1979
+    assert applied["track_number"] == 1
+    assert applied["duration_seconds"] == 222
+    assert applied["metadata_status"] == "enriched"
+
+
+async def test_manual_track_is_never_touched() -> None:
+    track = _track(metadata_status="manual")
+    service, tracks, _, _, _ = _service(track=track, tags=AudioTags(artist="X"))
+
+    result = await service.enrich(track.id)
+
+    assert result.status == "skipped"
+    assert tracks.applied is None  # nothing written
+
+
+async def test_missing_track_is_a_clean_noop() -> None:
+    service, tracks, _, _, _ = _service(track=None)
+
+    result = await service.enrich(uuid.uuid4())
+
+    assert result.status == "skipped"
+    assert tracks.applied is None
+
+
+async def test_nothing_found_marks_failed() -> None:
+    track = _track()
+    # No tags, no fingerprint → no identity at all.
+    service, tracks, artists, albums, _acoustid = _service(track=track, tags=None, fp=None)
+
+    result = await service.enrich(track.id)
+
+    assert result.status == "failed"
+    assert artists.created == []  # artist stays the original unknown
+    assert albums.created == []
+    applied = tracks.applied
+    assert applied is not None
+    assert applied["artist_id"] == track.artist_id  # fallback kept
+    assert applied["metadata_status"] == "failed"
+
+
+async def test_acoustid_path_fills_when_tags_absent() -> None:
+    track = _track()
+    fp = Fingerprint(fingerprint="AQAAxyz", duration_seconds=200)
+    match = RecordingMatch(
+        acoustid="acoustid-uuid",
+        score=0.95,
+        recording_mbid="mb-recording-id",
+        title="Identified Title",
+        artist="Daft Punk",
+        album="Discovery",
+    )
+    service, tracks, artists, _albums, acoustid = _service(
+        track=track, tags=None, fp=fp, match=match
+    )
+
+    result = await service.enrich(track.id)
+
+    assert result.status == "enriched"
+    assert result.matched_mbid == "mb-recording-id"
+    assert acoustid.calls == 1
+    applied = tracks.applied
+    assert applied is not None
+    assert applied["title"] == "Identified Title"
+    assert applied["musicbrainz_id"] == "mb-recording-id"
+    assert applied["acoustid_fingerprint"] == "acoustid-uuid"
+    assert "Daft Punk" in artists.created
+
+
+async def test_tags_win_over_acoustid_for_overlapping_fields() -> None:
+    track = _track()
+    fp = Fingerprint(fingerprint="AQAA", duration_seconds=200)
+    tags = AudioTags(title="Tagged Title", artist="Tagged Artist")
+    match = RecordingMatch(
+        acoustid="aid",
+        score=0.9,
+        recording_mbid="mbid",
+        title="AcoustID Title",
+        artist="AcoustID Artist",
+    )
+    service, tracks, artists, _albums, _acoustid = _service(
+        track=track, tags=tags, fp=fp, match=match
+    )
+
+    await service.enrich(track.id)
+
+    applied = tracks.applied
+    assert applied is not None
+    assert applied["title"] == "Tagged Title"  # tag preferred
+    assert "Tagged Artist" in artists.created
+    # but the MBID from AcoustID is still captured
+    assert applied["musicbrainz_id"] == "mbid"
+
+
+async def test_fingerprint_skipped_when_acoustid_unavailable() -> None:
+    track = _track()
+    fp = Fingerprint(fingerprint="AQAA", duration_seconds=200)
+    service, _tracks, _artists, _albums, acoustid = _service(
+        track=track, tags=AudioTags(artist="Tagged"), fp=fp, acoustid_available=False
+    )
+
+    result = await service.enrich(track.id)
+
+    # tags still enrich, but no AcoustID call is attempted
+    assert acoustid.calls == 0
+    assert result.status == "enriched"