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/api/chapters.rs

@@ -146,14 +146,26 @@ async fn create(
             manga_id, chapter.id, nnnn, page.ext
         );
         state.storage.put(&key, &page.bytes).await?;
-        let created =
-            repo::page::create(&mut *tx, chapter.id, page_number, &key, page.mime).await?;
+        let created = repo::page::create(
+            &mut *tx,
+            chapter.id,
+            page_number,
+            &key,
+            page.mime,
+            page.bytes.len() as i64,
+        )
+        .await?;
         page_ids.push(created.id);
     }
 
     let page_count = pages.len() as i32;
     repo::chapter::set_page_count(&mut *tx, chapter.id, page_count).await?;
     chapter.page_count = page_count;
+    // `repo::chapter::create` returned the row before any pages existed, so
+    // its `size_bytes` is a stale 0. Every uploaded page's size was just
+    // captured, so the true total is the sum of their byte lengths — set it
+    // on the response so the 201 body matches the persisted state.
+    chapter.size_bytes = Some(pages.iter().map(|p| p.bytes.len() as i64).sum());
 
     tx.commit().await?;