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/features/online/OnlineDetail.tsx

@@ -129,7 +129,10 @@ export function OnlineDetail({ postId }: { postId: number }) {
           post: target,
         },
       }),
-    onSuccess: (task) => setDownloadTaskId(task.task_id),
+    onSuccess: (task) => {
+      setDownloadTaskId(task.task_id);
+      void queryClient.invalidateQueries({ queryKey: ["status"] });
+    },
   });
 
   const taskQuery = useQuery({
@@ -143,20 +146,31 @@ export function OnlineDetail({ postId }: { postId: number }) {
     },
   });
   const downloadTask = taskQuery.data;
+  const downloadStatus = downloadTask?.status;
   const downloadActive =
-    downloadTask?.status === "pending" || downloadTask?.status === "downloading";
+    downloadStatus === "pending" || downloadStatus === "downloading";
 
   useEffect(() => {
-    if (downloadTask?.status === "complete") {
+    if (!downloadStatus) return;
+    if (
+      downloadStatus === "complete" ||
+      downloadStatus === "error" ||
+      downloadStatus === "cancelled"
+    ) {
+      void queryClient.invalidateQueries({ queryKey: ["status"] });
+    }
+    if (downloadStatus === "complete") {
       void queryClient.invalidateQueries({ queryKey: ["library-lookup"] });
       void queryClient.invalidateQueries({ queryKey: ["files"] });
     }
-  }, [downloadTask?.status, queryClient]);
+  }, [downloadStatus, queryClient]);
 
   function cancelDownload() {
     if (!downloadTaskId) return;
     void api(`/api/online/downloads/${downloadTaskId}/cancel/`, {
       method: "POST",
+    }).then(() => {
+      void queryClient.invalidateQueries({ queryKey: ["status"] });
     });
   }