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/api/admin/mangas.rs

@@ -25,6 +25,18 @@ pub fn routes() -> Router<AppState> {
         .route("/admin/mangas/:id/chapters", get(list_chapters))
 }
 
+#[derive(Debug, Deserialize, Default)]
+pub struct ListChaptersParams {
+    #[serde(default = "default_chapter_limit")]
+    pub limit: i64,
+    #[serde(default)]
+    pub offset: i64,
+}
+
+fn default_chapter_limit() -> i64 {
+    200
+}
+
 #[derive(Debug, Deserialize, Default)]
 pub struct ListMangasParams {
     #[serde(default)]
@@ -76,12 +88,23 @@ async fn list_chapters(
     State(state): State<AppState>,
     _admin: RequireAdmin,
     Path(manga_id): Path<Uuid>,
-) -> AppResult<Json<Vec<AdminChapterRow>>> {
+    Query(params): Query<ListChaptersParams>,
+) -> AppResult<Json<PagedResponse<AdminChapterRow>>> {
     // Explicit existence check so a typo / deleted manga returns 404
     // rather than a misleading "no chapters" 200.
     if !repo::manga::exists(&state.db, manga_id).await? {
         return Err(AppError::NotFound);
     }
-    let rows = repo::admin_view::list_chapters_with_sync_state(&state.db, manga_id).await?;
-    Ok(Json(rows))
+    // Cap at 500 to bound the per-row scalar-subquery cost on
+    // long-runners with thousands of chapters; default 200 covers
+    // typical browsing without paging round-trips.
+    let limit = params.limit.clamp(1, 500);
+    let offset = params.offset.max(0);
+    let q = repo::admin_view::ListAdminChaptersQuery {
+        manga_id,
+        limit,
+        offset,
+    };
+    let (items, total) = repo::admin_view::list_chapters_with_sync_state(&state.db, &q).await?;
+    Ok(Json(PagedResponse::with_total(items, limit, offset, total)))
 }