Live worker counts and an e621 request-history dropdown

- Status polling drops from 30s to 5s, and download start/finish/cancel
  invalidates it immediately, so the footer's Active Workers reflects
  running download tasks in near real time
- The e621 time in the status pill is now a button: it opens a dropdown
  with the session's request history (clock time, endpoint, duration,
  colour-coded) plus totals; closes on outside click or Escape
- Metrics store keeps the last 50 requests

frontend/src/components/AppShell.tsx

@@ -28,8 +28,8 @@ export function AppShell() {
   const statusQuery = useQuery({
     queryKey: ["status"],
     queryFn: () => api<SystemStatus>("/api/status/"),
-    refetchInterval: 30_000,
-    staleTime: 15_000,
+    refetchInterval: 5_000,
+    staleTime: 4_000,
   });
   const status = statusQuery.data;