Nodes: Update All Nodes — fleet-wide update request for every approved node

POST /nodes/request-update-all (admin) does what Update Node does for each
approved node: flags update_requested_at for the HTTPS updater and nudges it
over MQTT. Pending and rejected nodes are skipped. Admin-only button on the
Nodes page reports which nodes weren't reachable over MQTT.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-frontend/lib/c2api.ts

@@ -114,6 +114,12 @@ export const c2api = {
       `/nodes/${id}/request-update`,
       { method: "POST" },
     ),
+  /** Fleet-wide Update Node: every approved node; pending/rejected are skipped. */
+  requestUpdateAllNodes: () =>
+    request<{ ok: boolean; update_requested_at: string; requested: string[]; mqtt_sent: string[]; skipped: string[] }>(
+      `/nodes/request-update-all`,
+      { method: "POST" },
+    ),
   rejectNode: (id: string) =>
     request(`/nodes/${id}/reject`, { method: "POST" }),