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>

frontend/src/lib/api/admin.ts

@@ -180,6 +180,61 @@ export async function getSystemStats(): Promise<SystemStats> {
     return request<SystemStats>('/v1/admin/system');
 }
 
+// ---- storage usage ---------------------------------------------------------
+
+export type TopManga = {
+    id: string;
+    title: string;
+    total_bytes: number;
+};
+
+export type TopChapter = {
+    id: string;
+    manga_id: string;
+    number: number;
+    title: string | null;
+    total_bytes: number;
+};
+
+export type StorageStats = {
+    total_bytes: number;
+    covers_bytes: number;
+    chapters_bytes: number;
+    disk_total_bytes: number | null;
+    ratio_of_disk: number | null;
+    avg_image_bytes: number | null;
+    avg_cover_bytes: number | null;
+    avg_chapter_page_bytes: number | null;
+    top_mangas: TopManga[];
+    top_chapters: TopChapter[];
+    /** Pages still awaiting a size backfill. When > 0 the figures above are
+     * partial and the leaderboards omit not-yet-measured content. */
+    unmeasured_pages: number;
+    /** Cover blobs still awaiting a size backfill. */
+    unmeasured_covers: number;
+};
+
+export type StorageBackfillResult = {
+    pages: number;
+    covers: number;
+    /** Blobs storage reports as absent (orphaned). */
+    missing: number;
+    /** Blobs that errored for another reason (bad key, transient IO) — retryable. */
+    errored: number;
+    /** `true` when a per-run cap was hit before the backlog drained; run again. */
+    more_remaining: boolean;
+};
+
+export async function getStorageStats(): Promise<StorageStats> {
+    return request<StorageStats>('/v1/admin/storage');
+}
+
+export async function backfillStorage(): Promise<StorageBackfillResult> {
+    return request<StorageBackfillResult>('/v1/admin/storage/backfill', {
+        method: 'POST'
+    });
+}
+
 // ---- force resync ----------------------------------------------------------
 
 export type MangaResyncResponse = {