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.

backend/src/repo/admin_view.rs

@@ -166,11 +166,23 @@ pub struct AdminChapterRow {
     pub latest_seen_at: Option<DateTime<Utc>>,
 }
 
+#[derive(Debug, Default)]
+pub struct ListAdminChaptersQuery {
+    pub manga_id: Uuid,
+    pub limit: i64,
+    pub offset: i64,
+}
+
+/// Paginated chapter list with derived sync state. Pagination is non-
+/// optional — long-runners can have thousands of chapters and the
+/// per-row scalar subqueries make the unbounded variant a real
+/// stall risk even behind an admin guard. Returns the page slice plus
+/// the unfiltered total so the UI can render "showing N of M".
 pub async fn list_chapters_with_sync_state(
     pool: &PgPool,
-    manga_id: Uuid,
-) -> AppResult<Vec<AdminChapterRow>> {
-    let rows: Vec<AdminChapterRow> = sqlx::query_as(
+    q: &ListAdminChaptersQuery,
+) -> AppResult<(Vec<AdminChapterRow>, i64)> {
+    let items: Vec<AdminChapterRow> = sqlx::query_as(
         r#"
         SELECT
             c.id, c.manga_id, c.number, c.title, c.page_count, c.created_at,
@@ -202,10 +214,19 @@ pub async fn list_chapters_with_sync_state(
           FROM chapters c
          WHERE c.manga_id = $1
          ORDER BY c.number ASC
+         LIMIT $2 OFFSET $3
         "#,
     )
-    .bind(manga_id)
+    .bind(q.manga_id)
+    .bind(q.limit)
+    .bind(q.offset)
     .fetch_all(pool)
     .await?;
-    Ok(rows)
+
+    let total: i64 = sqlx::query_scalar("SELECT COUNT(*) FROM chapters WHERE manga_id = $1")
+        .bind(q.manga_id)
+        .fetch_one(pool)
+        .await?;
+
+    Ok((items, total))
 }