fix(admin): security-audit findings — paginate chapters, lock down unchecked helper (0.41.2)

Addresses the security-audit findings on top of the admin feature stack:

M1: /admin/mangas/:id/chapters now paginates (default limit 200, max 500).
A long-runner with thousands of chapters would otherwise produce a multi-MB
response with that many scalar subqueries per row — admin-only but a real
stall risk on one expand-click. Adds explicit pagination tests for the cap
and offset; frontend renders a "Showing first N of M" hint when the cap
clips the result.

L1: repo::user::set_is_admin renamed to set_is_admin_unchecked with a
doc-comment pointing at admin_safe_set_is_admin for production use. The
short name was a footgun — a future contributor reaching for it would
silently bypass self-protection, the last-admin invariant, and the audit
log. Used only by integration-test setup; production code goes through
the admin_safe_* paths.

CSRF posture: build_session_cookie carries a comment that the
SameSite=Lax default is the project's CSRF defense for state-changing
mutations and breaks the instant anyone adds a side-effecting GET under
/admin/*. Spells out what to do then (Strict + explicit token check).

Test counts: 43 backend admin tests + 12 vitest admin tests all green;
svelte-check 0/0 across 446 files.

frontend/src/routes/admin/mangas/+page.svelte

@@ -14,7 +14,11 @@
     let syncFilter: MangaSyncState | '' = $state('');
     let error: string | null = $state(null);
     let expandedId: string | null = $state(null);
-    let chaptersByManga: Record<string, AdminChapterRow[] | 'loading'> = $state({});
+    type ChaptersView = {
+        items: AdminChapterRow[];
+        total: number;
+    };
+    let chaptersByManga: Record<string, ChaptersView | 'loading'> = $state({});
 
     async function load() {
         error = null;
@@ -40,7 +44,11 @@
         if (!chaptersByManga[id]) {
             chaptersByManga[id] = 'loading';
             try {
-                chaptersByManga[id] = await listAdminChapters(id);
+                const resp = await listAdminChapters(id, { limit: 500 });
+                chaptersByManga[id] = {
+                    items: resp.items,
+                    total: resp.page.total ?? resp.items.length
+                };
             } catch {
                 delete chaptersByManga[id];
                 error = 'failed to load chapters';
@@ -113,10 +121,16 @@
                             {#if chaptersByManga[m.id] === 'loading'}
                                 <p>Loading chapters…</p>
                             {:else if chaptersByManga[m.id]}
-                                {@const list = chaptersByManga[m.id] as AdminChapterRow[]}
-                                {#if list.length === 0}
+                                {@const view = chaptersByManga[m.id] as ChaptersView}
+                                {#if view.items.length === 0}
                                     <p class="muted">No chapters.</p>
                                 {:else}
+                                    {#if view.total > view.items.length}
+                                        <p class="muted">
+                                            Showing first {view.items.length} of {view.total}
+                                            chapters (cap reached).
+                                        </p>
+                                    {/if}
                                     <table class="inner">
                                         <thead>
                                             <tr>
@@ -127,7 +141,7 @@
                                             </tr>
                                         </thead>
                                         <tbody>
-                                            {#each list as c (c.id)}
+                                            {#each view.items as c (c.id)}
                                                 <tr>
                                                     <td>{c.number}</td>
                                                     <td>{c.title ?? '—'}</td>