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/tests/api_chapters.rs

@@ -45,6 +45,80 @@ async fn list_chapters_is_empty_initially(pool: PgPool) {
     assert!(body["page"]["total"].is_null());
 }
 
+#[sqlx::test(migrations = "./migrations")]
+async fn uncrawled_chapter_reports_zero_storage(pool: PgPool) {
+    // A page-less chapter (page_count = 0, no page rows) is "uncrawled"
+    // — its size_bytes is 0, which the frontend renders as an em-dash.
+    let h = common::harness(pool.clone());
+    let (_, cookie) = common::register_user(&h.app).await;
+    let manga_id = seed_manga(&h, &cookie, "Berserk").await;
+    seed_chapter(&pool, manga_id, 1, Some("Unfetched")).await;
+
+    let resp = h
+        .app
+        .oneshot(common::get(&format!("/api/v1/mangas/{manga_id}/chapters")))
+        .await
+        .unwrap();
+    let body = common::body_json(resp).await;
+    assert_eq!(body["items"][0]["page_count"].as_i64().unwrap(), 0);
+    assert_eq!(body["items"][0]["size_bytes"].as_i64().unwrap(), 0);
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn chapter_with_unmeasured_page_reports_null_storage(pool: PgPool) {
+    // A crawled chapter whose pages predate the size backfill (size_bytes
+    // IS NULL) must report size_bytes = null — "unknown" — so the frontend
+    // shows an em-dash, not a misleading "0 B".
+    let h = common::harness(pool.clone());
+    let (_, cookie) = common::register_user(&h.app).await;
+    let manga_id = seed_manga(&h, &cookie, "Berserk").await;
+    let chapter_id = seed_chapter(&pool, manga_id, 1, Some("Crawled")).await;
+    // One measured page, one unmeasured (NULL) → whole chapter is unknown.
+    sqlx::query(
+        "INSERT INTO pages (chapter_id, page_number, storage_key, content_type, size_bytes) \
+         VALUES ($1, 1, 'k/1.jpg', 'image/jpeg', 100)",
+    )
+    .bind(chapter_id)
+    .execute(&pool)
+    .await
+    .unwrap();
+    sqlx::query(
+        "INSERT INTO pages (chapter_id, page_number, storage_key, content_type) \
+         VALUES ($1, 2, 'k/2.jpg', 'image/jpeg')",
+    )
+    .bind(chapter_id)
+    .execute(&pool)
+    .await
+    .unwrap();
+
+    let resp = h
+        .app
+        .clone()
+        .oneshot(common::get(&format!("/api/v1/mangas/{manga_id}/chapters")))
+        .await
+        .unwrap();
+    let body = common::body_json(resp).await;
+    assert!(
+        body["items"][0]["size_bytes"].is_null(),
+        "unmeasured page should make the chapter size unknown (null), got {}",
+        body["items"][0]["size_bytes"]
+    );
+
+    // The manga detail total is likewise unknown (null), not a silent
+    // undercount of just the measured page.
+    let detail = h
+        .app
+        .oneshot(common::get(&format!("/api/v1/mangas/{manga_id}")))
+        .await
+        .unwrap();
+    let detail = common::body_json(detail).await;
+    assert!(
+        detail["chapter_storage_bytes"].is_null(),
+        "manga total should be unknown when a page is unmeasured, got {}",
+        detail["chapter_storage_bytes"]
+    );
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn list_chapters_returned_in_number_order(pool: PgPool) {
     let h = common::harness(pool.clone());