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/routes/manga/[id]/+page.svelte

@@ -13,6 +13,7 @@
     } from '$lib/api/mangas';
     import { resyncManga } from '$lib/api/admin';
     import { chapterLabel } from '$lib/api/chapters';
+    import { formatBytes } from '$lib/upload-validation';
     import { listTags, type Tag } from '$lib/api/tags';
     import type { ContentWarning } from '$lib/api/page_tags';
     import { session } from '$lib/session.svelte';
@@ -39,6 +40,12 @@
     const manga = $derived<MangaDetail>(mangaOverride ?? data.manga);
     const chapters = $derived(data.chapters);
     const readProgress = $derived(data.readProgress);
+    /** Total stored bytes of this manga's chapter content (cover excluded).
+     *  Comes from the backend, which sums over ALL chapters (the loaded
+     *  chapter list is paginated, so summing it client-side would
+     *  undercount mangas with many chapters). `null` → unknown (a page is
+     *  un-backfilled) → show an em-dash; `0` → no content → hide. */
+    const contentBytes = $derived(manga.chapter_storage_bytes);
     /** Chapter row from the local chapters list when present (so we
      *  can also surface the chapter title). Falls back below to the
      *  server-supplied `chapter_number` when the chapter sits past
@@ -589,7 +596,14 @@
     {/if}
 
     <section aria-label="chapters">
-        <h2>Chapters</h2>
+        <div class="chapters-head">
+            <h2>Chapters</h2>
+            {#if contentBytes === null || contentBytes > 0}
+                <span class="content-size" data-testid="manga-content-size">
+                    Content: {contentBytes === null ? '—' : formatBytes(contentBytes)}
+                </span>
+            {/if}
+        </div>
         {#if continueChapterId != null && continueChapterNumber != null}
             <a
                 class="continue"
@@ -615,6 +629,11 @@
                             {chapterLabel(c)}
                         </a>
                         <span class="pages">({c.page_count} pages)</span>
+                        <span class="size" data-testid="chapter-size">
+                            {c.page_count === 0 || c.size_bytes === null
+                                ? '—'
+                                : formatBytes(c.size_bytes)}
+                        </span>
                     </li>
                 {/each}
             </ol>
@@ -996,6 +1015,26 @@
         font-size: var(--font-sm);
     }
 
+    .chapters-head {
+        display: flex;
+        align-items: baseline;
+        justify-content: space-between;
+        gap: var(--space-3);
+    }
+
+    .content-size {
+        color: var(--text-muted);
+        font-size: var(--font-sm);
+        font-family: var(--font-mono, monospace);
+    }
+
+    .size {
+        color: var(--text-muted);
+        margin-left: var(--space-2);
+        font-size: var(--font-sm);
+        font-family: var(--font-mono, monospace);
+    }
+
     /* Mobile hero + chrome — hidden above 640px so the existing
        desktop layout stays untouched. */
     .mobile-hero {