feat: implement traffic insight marker backend (ubridge mark filter)

Add compute-side marker subsystem that receives ubridge UDP MARK signals
and forwards them as project-scoped notifications to the web UI for
real-time traffic coloring. Matched packets are always saved to per-link
pcaps for future replay.

Key components:
- gns3server/compute/marker/: MarkerManager (singleton, UDP listener +
  O(1) registry keyed by (node_id, filter_name)) + MarkerListener
  (DatagramProtocol parsing MARK lines per ubridge integration contract)
- gns3server/compute/base_node.py: marker sink/node config at ubridge
  startup; shared _ubridge_add_marker_filter / _ubridge_delete_marker_filter
- Per-node start_marker/stop_marker: VPCS (VPCS-{id}), QEMU
  (QEMU-{id}-{adapter}), Docker (bridge{adapter})
- Compute REST /markers/start + /markers/stop (vpcs/qemu/docker route files)
- Controller Link._markers state + UDPLink.start_marker/stop_marker/
  update_marker (mirror capture pattern: BPF validation, _choose_capture_side,
  node.post forwarding, topology persistence)
- Controller REST GET/POST/DELETE/PUT /v3/projects/{p}/links/{l}/markers
- Config: marker_listen_host / marker_listen_port in ServerSettings
- Signal routing: creation-time registry O(1) lookup, no node-table scan;
  project-scoped WS stream (not global); event payload always carries
  project_id for frontend scoping

Tests: 14 unit tests (registry, listener parsing, UDP round-trip);
562 existing tests pass with zero regressions.

gns3server/core/tasks.py

@@ -24,6 +24,7 @@ from gns3server.controller import Controller
 from gns3server.config import Config
 from gns3server.compute import MODULES
 from gns3server.compute.port_manager import PortManager
+from gns3server.compute.marker.marker_manager import MarkerManager
 from gns3server.utils.http_client import HTTPClient
 from gns3server.db.tasks import connect_to_db, get_computes, disconnect_from_db, discover_images_on_filesystem
 
@@ -84,6 +85,14 @@ async def startup(app: FastAPI) -> None:
         m = module.instance()
         m.port_manager = PortManager.instance()
 
+    # Start the marker (traffic-insight) UDP sink. One listener per compute
+    # process receives ubridge MARK signals; ubridges are told its host/port at
+    # startup (see BaseNode._start_ubridge).
+    server_settings = Config.instance().settings.Server
+    await MarkerManager.instance().start(
+        host=server_settings.marker_listen_host, port=server_settings.marker_listen_port
+    )
+
     # Mark MCP server as ready to accept connections (if MCP is available)
     from gns3server.agent import MCP_AVAILABLE
 
@@ -101,6 +110,7 @@ async def shutdown(app: FastAPI) -> None:
     if auto_discover_images_task_handle is not None and not auto_discover_images_task_handle.cancelled():
         auto_discover_images_task_handle.cancel()
     await HTTPClient.close_session()
+    await MarkerManager.instance().stop()
     await Controller.instance().stop()
 
     for module in MODULES: