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/repo/page.rs

@@ -12,11 +12,12 @@ pub async fn create<'e, E: PgExecutor<'e>>(
     page_number: i32,
     storage_key: &str,
     content_type: &str,
+    size_bytes: i64,
 ) -> AppResult<Page> {
     let row = sqlx::query_as::<_, Page>(
         r#"
-        INSERT INTO pages (chapter_id, page_number, storage_key, content_type)
-        VALUES ($1, $2, $3, $4)
+        INSERT INTO pages (chapter_id, page_number, storage_key, content_type, size_bytes)
+        VALUES ($1, $2, $3, $4, $5)
         RETURNING id, chapter_id, page_number, storage_key, content_type
         "#,
     )
@@ -24,6 +25,7 @@ pub async fn create<'e, E: PgExecutor<'e>>(
     .bind(page_number)
     .bind(storage_key)
     .bind(content_type)
+    .bind(size_bytes)
     .fetch_one(executor)
     .await?;
     Ok(row)