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/lib/api/admin.ts

@@ -102,9 +102,26 @@ export type AdminChapterRow = {
     latest_seen_at: string | null;
 };
 
-export async function listAdminChapters(mangaId: string): Promise<AdminChapterRow[]> {
-    return request<AdminChapterRow[]>(
-        `/v1/admin/mangas/${encodeURIComponent(mangaId)}/chapters`
+export type AdminChaptersPage = {
+    items: AdminChapterRow[];
+    page: Page;
+};
+
+export type ListAdminChaptersOptions = {
+    limit?: number;
+    offset?: number;
+};
+
+export async function listAdminChapters(
+    mangaId: string,
+    opts: ListAdminChaptersOptions = {}
+): Promise<AdminChaptersPage> {
+    const params = new URLSearchParams();
+    if (opts.limit != null) params.set('limit', String(opts.limit));
+    if (opts.offset != null) params.set('offset', String(opts.offset));
+    const qs = params.toString();
+    return request<AdminChaptersPage>(
+        `/v1/admin/mangas/${encodeURIComponent(mangaId)}/chapters${qs ? `?${qs}` : ''}`
     );
 }