Add rate limiting dashboard to admin panel

- Enhance GET /admin/rate-limits with per-IP breakdown, 24h throttled
  count, and hourly time-series of rejected requests
- Add _rejected_log deque for time-series tracking of throttled requests
- Add AdminRateLimits React page with auto-refresh (configurable 15s/30s/1m),
  summary cards, throttled-over-time bar chart, endpoint table, per-IP table
- Add TypeScript types (RateLimitStatsResponse) and adminApi.getRateLimits()
- Wire up /admin/rate-limits route and nav link (admin-only)
- Expand unit tests to 10 cases: auth, empty state, per-IP breakdown,
  throttled_24h count, time-series structure, response shape contract

Closes leeworks-agents/SPARC#1686

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -11,6 +11,7 @@ import { Batch } from './pages/Batch';
 import { AnalyticsPage } from './pages/Analytics';
 import { About } from './pages/About';
 import { AdminUsers } from './pages/AdminUsers';
+import { AdminRateLimits } from './pages/AdminRateLimits';
 import { Compare } from './pages/Compare';
 
 const queryClient = new QueryClient({
@@ -56,6 +57,14 @@ function App() {
                   </ProtectedRoute>
                 }
               />
+              <Route
+                path="/admin/rate-limits"
+                element={
+                  <ProtectedRoute requireAdmin>
+                    <AdminRateLimits />
+                  </ProtectedRoute>
+                }
+              />
             </Route>
 
             {/* Default redirect */}