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>

app/application/import_service.py

@@ -9,7 +9,7 @@ must not abort the whole scan (graceful degradation).
 
 import contextlib
 import uuid
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 
 from app.core.logging import get_logger
 from app.domain.ports import ArtistRepository, FileStorage, IndexableSource, TrackRepository
@@ -27,6 +27,9 @@ class ImportSummary:
     imported: int
     skipped: int
     failed: int
+    # IDs of freshly imported tracks, for the caller to enqueue enrichment
+    # *after* its transaction commits (enqueuing mid-scan would race the worker).
+    imported_ids: list[uuid.UUID] = field(default_factory=list)
 
 
 class LibraryImportService:
@@ -44,7 +47,8 @@ class LibraryImportService:
     async def scan_and_import(
         self, source: IndexableSource, *, added_by: uuid.UUID | None
     ) -> ImportSummary:
-        seen = imported = skipped = failed = 0
+        seen = skipped = failed = 0
+        imported_ids: list[uuid.UUID] = []
         for file in source.scan():
             seen += 1
             try:
@@ -52,13 +56,18 @@ class LibraryImportService:
                 if existing is not None:
                     skipped += 1
                     continue
-                await self._import_one(source.name, file, added_by)
-                imported += 1
+                track_id = await self._import_one(source.name, file, added_by)
+                imported_ids.append(track_id)
             except Exception:
                 failed += 1
                 log.warning("import_file_failed", source=source.name, source_id=file.source_id)
         summary = ImportSummary(
-            source=source.name, seen=seen, imported=imported, skipped=skipped, failed=failed
+            source=source.name,
+            seen=seen,
+            imported=len(imported_ids),
+            skipped=skipped,
+            failed=failed,
+            imported_ids=imported_ids,
         )
         log.info(
             "import_complete",
@@ -72,7 +81,7 @@ class LibraryImportService:
 
     async def _import_one(
         self, source_name: str, file: SourceFile, added_by: uuid.UUID | None
-    ) -> None:
+    ) -> uuid.UUID:
         track_id = uuid.uuid4()
         key = f"tracks/{str(track_id)[:2]}/{track_id}.{file.file_format}"
         await self._storage.save_file(key, file.path)
@@ -94,3 +103,4 @@ class LibraryImportService:
             with contextlib.suppress(Exception):
                 await self._storage.delete(key)
             raise
+        return track_id