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.test.ts

@@ -15,6 +15,8 @@ import {
     listAdminMangas,
     listAdminChapters,
     getSystemStats,
+    getStorageStats,
+    backfillStorage,
     resyncManga,
     resyncChapter,
     getCrawlerStatus,
@@ -587,4 +589,66 @@ describe('admin crawler api client', () => {
             /\/v1\/admin\/analysis\/status\/stream$/
         );
     });
+
+    // ---- storage usage ----
+
+    it('getStorageStats GETs /v1/admin/storage and parses the envelope', async () => {
+        const fixture = {
+            total_bytes: 700,
+            covers_bytes: 100,
+            chapters_bytes: 600,
+            disk_total_bytes: 1_000_000,
+            ratio_of_disk: 0.0007,
+            avg_image_bytes: 233.3,
+            avg_cover_bytes: 100,
+            avg_chapter_page_bytes: 300,
+            top_mangas: [{ id: 'm-1', title: 'Big', total_bytes: 650 }],
+            top_chapters: [
+                { id: 'c-1', manga_id: 'm-1', number: 1, title: null, total_bytes: 600 }
+            ],
+            unmeasured_pages: 4,
+            unmeasured_covers: 1
+        };
+        fetchSpy.mockResolvedValueOnce(ok(fixture));
+        const s = await getStorageStats();
+        expect(s.total_bytes).toBe(700);
+        expect(s.top_mangas[0].title).toBe('Big');
+        expect(s.top_chapters[0].total_bytes).toBe(600);
+        expect(s.unmeasured_pages).toBe(4);
+        const url = fetchSpy.mock.calls[0][0] as string;
+        expect(url).toMatch(/\/v1\/admin\/storage$/);
+    });
+
+    it('getStorageStats keeps disk total + ratio null for a non-local store', async () => {
+        fetchSpy.mockResolvedValueOnce(
+            ok({
+                total_bytes: 0,
+                covers_bytes: 0,
+                chapters_bytes: 0,
+                disk_total_bytes: null,
+                ratio_of_disk: null,
+                avg_image_bytes: null,
+                avg_cover_bytes: null,
+                avg_chapter_page_bytes: null,
+                top_mangas: [],
+                top_chapters: [],
+                unmeasured_pages: 0,
+                unmeasured_covers: 0
+            })
+        );
+        const s = await getStorageStats();
+        expect(s.disk_total_bytes).toBeNull();
+        expect(s.ratio_of_disk).toBeNull();
+    });
+
+    it('backfillStorage POSTs /v1/admin/storage/backfill and returns counts', async () => {
+        fetchSpy.mockResolvedValueOnce(
+            ok({ pages: 3, covers: 1, missing: 0, errored: 0, more_remaining: false })
+        );
+        const r = await backfillStorage();
+        expect(r).toEqual({ pages: 3, covers: 1, missing: 0, errored: 0, more_remaining: false });
+        const url = fetchSpy.mock.calls[0][0] as string;
+        expect(url).toMatch(/\/v1\/admin\/storage\/backfill$/);
+        expect(fetchSpy.mock.calls[0][1]!.method).toBe('POST');
+    });
 });