feat: real-time WebSocket metrics broadcaster and UI updates

- New metrics_broadcaster service (410 lines) — background broadcaster
  pushing CPU/RAM/disk/network stats to subscribed WebSocket clients
- WebSocket protocol extended: subscribe/unsubscribe channels,
  get_rrd/get_node_rrd requests with request_id
- main.py: metrics broadcaster started in lifespan, WS message dispatcher
- websocket_manager.py: subscription map support
- templates: dashboard, instance_detail, proxmox_server_detail,
  proxmox_vms, workspaces — real-time charts and UI updates
- auth.py, servers.py, settings.py: minor fixes

backend/app/templates/backups.html

@@ -286,7 +286,7 @@
       </div>
       <div class="modal-body p-0">
         <div id="taskLogStatus" class="px-3 pt-3"></div>
-        <pre id="taskLogContent" style="max-height:400px;overflow-y:auto;background:var(--bs-dark,#1e1e2e);color:#cdd6f4;padding:1rem;margin:0;font-size:.82rem;"></pre>
+        <pre id="taskLogContent" style="max-height:400px;overflow-y:auto;background:var(--bg-surface-solid);color:var(--text-primary);padding:1rem;margin:0;font-size:.82rem;"></pre>
       </div>
     </div>
   </div>