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/api/schemas/storage.py

@@ -0,0 +1,45 @@
+"""Storage / library statistics response schemas (§A6)."""
+
+import datetime as dt
+
+from pydantic import BaseModel
+
+
+class DiskUsageOut(BaseModel):
+    total: int
+    used: int
+    free: int
+
+
+class FormatBreakdownOut(BaseModel):
+    file_format: str
+    track_count: int
+    total_size: int
+
+
+class GenreCountOut(BaseModel):
+    genre: str
+    track_count: int
+
+
+class StorageStatsOut(BaseModel):
+    """Everything the Storage screen needs in a single call."""
+
+    # library catalogue
+    total_tracks: int
+    total_artists: int
+    total_albums: int
+    total_size: int
+    total_duration_seconds: int
+    largest_track_size: int
+    earliest_added: dt.datetime | None
+    latest_added: dt.datetime | None
+
+    # breakdowns
+    by_format: list[FormatBreakdownOut]
+    by_metadata_status: dict[str, int]
+    by_source: dict[str, int]
+    top_genres: list[GenreCountOut]
+
+    # backing volume (``None`` for object-store backends)
+    disk: DiskUsageOut | None