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/ports.py

@@ -17,7 +17,9 @@ from app.domain.entities import (
     AudioTags,
     CoverArt,
     Credentials,
+    DiskUsage,
     Fingerprint,
+    LibraryStats,
     Like,
     ObjectStat,
     PlayHistoryEntry,
@@ -98,6 +100,10 @@ class FileStorage(Protocol):
     async def exists(self, key: str) -> bool: ...
     async def delete(self, key: str) -> None: ...
     def as_local_path(self, key: str) -> AbstractAsyncContextManager[Path]: ...
+    async def disk_usage(self) -> DiskUsage | None:
+        """Capacity of the volume backing the store, or ``None`` when the
+        backend has no addressable disk (e.g. an object store)."""
+        ...
 
 
 class ArtistRepository(Protocol):
@@ -128,9 +134,11 @@ class TrackRepository(Protocol):
         added_by: uuid.UUID | None,
     ) -> Track: ...
     async def delete(self, track_id: uuid.UUID) -> None: ...
-    # genres must come before ``list`` — the method named ``list`` shadows the
-    # builtin in later annotations (same pattern as AlbumRepository below).
+    # genres / library_stats must come before ``list`` — the method named
+    # ``list`` shadows the builtin in later annotations (same pattern as
+    # AlbumRepository below).
     async def genres(self) -> list[tuple[str, int]]: ...
+    async def library_stats(self) -> LibraryStats: ...
     async def list(
         self,
         *,