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/upload_history.rs

@@ -29,6 +29,7 @@ struct ChapterUploadRow {
     title: Option<String>,
     page_count: i32,
     created_at: chrono::DateTime<chrono::Utc>,
+    size_bytes: Option<i64>,
 }
 
 /// Returns up to `limit` of the user's most recent uploads (mangas and
@@ -64,7 +65,13 @@ pub async fn list_for_user(
                c.number,
                c.title,
                c.page_count,
-               c.created_at
+               c.created_at,
+               (SELECT CASE
+                         WHEN count(*) = 0 THEN 0
+                         WHEN bool_or(p.size_bytes IS NULL) THEN NULL
+                         ELSE sum(p.size_bytes)
+                       END
+                  FROM pages p WHERE p.chapter_id = c.id)::bigint AS size_bytes
           FROM chapters c
           JOIN mangas m ON m.id = c.manga_id
          WHERE c.uploaded_by = $1
@@ -97,6 +104,7 @@ pub async fn list_for_user(
                 title: c.title,
                 page_count: c.page_count,
                 created_at: c.created_at,
+                size_bytes: c.size_bytes,
             },
             created_at,
         });