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/storage.py

@@ -1,5 +1,6 @@
 """Value objects for file storage."""
 
+import datetime as dt
 from dataclasses import dataclass
 
 
@@ -7,3 +8,39 @@ from dataclasses import dataclass
 class ObjectStat:
     size: int
     content_type: str | None
+
+
+@dataclass(frozen=True, slots=True)
+class DiskUsage:
+    """Capacity of the volume backing the media store. ``None`` for backends
+    (e.g. object stores) that expose no notion of total disk capacity."""
+
+    total: int
+    used: int
+    free: int
+
+
+@dataclass(frozen=True, slots=True)
+class FormatBreakdown:
+    """Per-container-format slice of the library (e.g. ``flac`` → 312 tracks)."""
+
+    file_format: str
+    track_count: int
+    total_size: int
+
+
+@dataclass(frozen=True, slots=True)
+class LibraryStats:
+    """Aggregate facts about everything the instance has stored. Computed from
+    the catalogue (DB), not the filesystem — ``total_size`` is the sum of the
+    recorded ``file_size`` of every track."""
+
+    total_tracks: int
+    total_size: int
+    total_duration_seconds: int
+    by_format: list[FormatBreakdown]
+    by_metadata_status: dict[str, int]
+    by_source: dict[str, int]
+    largest_track_size: int
+    earliest_added: dt.datetime | None
+    latest_added: dt.datetime | None