feat(storage): library + disk statistics endpoint (§A6)

Implement `GET /api/v1/storage`, replacing the stub. Returns aggregate
library facts (track/artist/album counts, total footprint, playtime,
per-format / per-source / metadata-status breakdowns, top genres) plus
the real capacity of the backing volume.

- domain: `LibraryStats`, `FormatBreakdown`, `DiskUsage` value objects
- ports: `FileStorage.disk_usage()` (local = shutil.disk_usage walking up
  to the nearest existing ancestor; S3 returns None — no fixed disk)
- repo: `TrackRepository.library_stats()` (single set of GROUP BYs)
- tests: storage stats API (auth, empty library, upload counting)

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

app/domain/entities/__init__.py

@@ -6,7 +6,12 @@ from app.domain.entities.history import PlayHistoryEntry
 from app.domain.entities.like import Like
 from app.domain.entities.metadata import AudioTags, Fingerprint, RecordingMatch
 from app.domain.entities.playlist import Playlist
-from app.domain.entities.storage import ObjectStat
+from app.domain.entities.storage import (
+    DiskUsage,
+    FormatBreakdown,
+    LibraryStats,
+    ObjectStat,
+)
 from app.domain.entities.track import Artist, Track
 from app.domain.entities.user import Credentials, SubsonicCredentials, User
 
@@ -16,7 +21,10 @@ __all__ = [
     "AudioTags",
     "CoverArt",
     "Credentials",
+    "DiskUsage",
     "Fingerprint",
+    "FormatBreakdown",
+    "LibraryStats",
     "Like",
     "ObjectStat",
     "PlayHistoryEntry",