feat(storage): admin storage-usage stats + per-manga/chapter sizes

Persist blob sizes (covers + chapter pages) and surface upload storage
usage to admins in two places:

- Admin System tab: total/covers/chapters totals, ratio of disk, average
  image sizes, and top-5 largest mangas/chapters leaderboards, behind a
  new GET /api/v1/admin/storage endpoint (separate from /admin/system so
  the cheap DB aggregates aren't coupled to its 250ms CPU sample).
- Manga detail page: total chapter-content size and per-chapter size,
  with an em-dash for uncrawled or not-yet-measured chapters.

Sizes are captured at write time (crawler put_stream return, uploaded
page/cover byte length) into nullable pages.size_bytes /
mangas.cover_size_bytes columns; NULL means "not yet measured", distinct
from a real 0. A one-shot, idempotent admin "Backfill sizes" action
(POST /api/v1/admin/storage/backfill) stats pre-existing blobs in
keyset-paginated, capped batches and reports more_remaining so a large
legacy library can be drained over multiple runs.

Aggregates and leaderboards treat NULL honestly (only fully-measured
entities are ranked); a dashboard banner flags unmeasured rows so partial
figures aren't read as complete. persist_pages now prunes stale page rows
on a shrinking re-crawl so totals and page_count stay consistent.

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

backend/src/domain/mod.rs

@@ -12,6 +12,7 @@ pub mod page_tag;
 pub mod patch;
 pub mod read_progress;
 pub mod session;
+pub mod storage_stats;
 pub mod sync_state;
 pub mod tag;
 pub mod upload_entry;
@@ -38,6 +39,7 @@ pub use page_tag::{
 pub use patch::Patch;
 pub use read_progress::{ReadProgress, ReadProgressForManga, ReadProgressSummary};
 pub use session::Session;
+pub use storage_stats::{StorageStats, TopChapter, TopManga};
 pub use sync_state::{ChapterSyncState, MangaSyncState};
 pub use tag::{Tag, TagRef};
 pub use upload_entry::UploadEntry;