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/storage/mod.rs

@@ -82,6 +82,14 @@ pub trait Storage: Send + Sync {
     async fn delete(&self, key: &str) -> Result<(), StorageError>;
     async fn exists(&self, key: &str) -> Result<bool, StorageError>;
 
+    /// Size in bytes of the blob at `key`, `NotFound` when it doesn't
+    /// exist. Cheap metadata lookup (local: `fs::metadata`; a future
+    /// `S3Storage`: HEAD object). Used by the cover-capture path and the
+    /// one-shot storage-size backfill — never on the hot read path, which
+    /// reads the stored `size_bytes` columns. Required (no default) so a
+    /// new backend can't silently skip it.
+    async fn size(&self, key: &str) -> Result<u64, StorageError>;
+
     /// Filesystem path the backend is rooted at, when introspectable.
     /// Returns `None` for backends that aren't a local filesystem (e.g.
     /// a future `S3Storage`). The admin system endpoint uses this to