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/api/auth.py

@@ -216,6 +216,26 @@ def login(login_data: LoginRequest, request: Request, db: Session = Depends(get_
     }
 
 
+@router.post("/api/auth/refresh", response_model=Token)
+def refresh_token(
+    current_user: User = Depends(get_current_user),
+    session_token: str = Depends(get_current_session_token),
+    db: Session = Depends(get_db)
+):
+    """Refresh JWT access token without re-login (extends session)"""
+    session_timeout = SecurityService.get_setting_int(db, "session_timeout_minutes", 60)
+    access_token_expires = timedelta(minutes=session_timeout)
+    new_token = create_access_token(
+        data={"sub": current_user.username},
+        session_token=session_token,
+        expires_delta=access_token_expires
+    )
+    return {
+        "access_token": new_token,
+        "token_type": "bearer"
+    }
+
+
 @router.post("/api/auth/logout")
 def logout(
     request: Request,