Paginate the Review Queue with cursor-based infinite scroll

Fetching every needs_review activity's laps and per-second samples up front
gets expensive once there are many -- that work is now deferred to the
current page only. GET /api/review-queue/ takes limit/before and returns
{items, next_cursor, total}; sorting/cursor filtering still needs each
activity's cheap summary row, but only the requested page's items get their
laps/samples fetched.

Frontend loads 10 at a time and fetches the next page when the list's
bottom sentinel scrolls into view. Selecting a specific-kind filter (not
"All") loads the rest of the backlog up front, since filtering only the
items scrolled into view so far would hide matches further down.

frontend/src/types/api.ts

@@ -140,6 +140,12 @@ export interface ReviewQueueItem extends KindAssignment {
   samples: Sample[];
 }
 
+export interface ReviewQueuePage {
+  items: ReviewQueueItem[];
+  next_cursor: string | null;
+  total: number;
+}
+
 export interface ProgressionPoint {
   date: string;
   activity_id: number;