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/api/routes/compute/docker_nodes.py

@@ -29,6 +29,7 @@ from typing import Union
 from gns3server import schemas
 from gns3server.compute.docker import Docker
 from gns3server.compute.docker.docker_vm import DockerVM
+from gns3server.compute.marker.marker_manager import MarkerManager
 
 from .dependencies.authentication import compute_authentication, ws_compute_authentication
 
@@ -353,6 +354,52 @@ async def stop_docker_node_capture(
     await node.stop_capture(adapter_number)
 
 
+@router.post(
+    "/{node_id}/adapters/{adapter_number}/ports/{port_number}/markers/start",
+    dependencies=[Depends(compute_authentication)]
+)
+async def start_docker_node_marker(
+    *,
+    project_id: UUID,
+    adapter_number: int,
+    port_number: int,
+    marker_data: schemas.MarkerCreate,
+    node: DockerVM = Depends(dep_node)
+) -> dict:
+    """
+    Attach a traffic-insight ``mark`` filter to the Docker node's uBridge bridge.
+    """
+
+    pcap_path = os.path.join(
+        node.project.markers_working_directory(),
+        f"{node.id}_{marker_data.link_id}_{marker_data.name}.pcap"
+    )
+    await node.start_marker(adapter_number, marker_data.name, marker_data.bpf, pcap_path, marker_data.tag)
+    MarkerManager.instance().register(
+        str(project_id), node.id, marker_data.name, marker_data.link_id, marker_data.tag
+    )
+    return {"pcap_file_path": str(pcap_path)}
+
+
+@router.post(
+    "/{node_id}/adapters/{adapter_number}/ports/{port_number}/markers/stop",
+    status_code=status.HTTP_204_NO_CONTENT,
+    dependencies=[Depends(compute_authentication)]
+)
+async def stop_docker_node_marker(
+    adapter_number: int,
+    port_number: int,
+    marker_data: schemas.MarkerDelete,
+    node: DockerVM = Depends(dep_node)
+) -> None:
+    """
+    Remove a traffic-insight ``mark`` filter from the Docker node's uBridge bridge.
+    """
+
+    await node.stop_marker(adapter_number, marker_data.name)
+    MarkerManager.instance().unregister(node.id, marker_data.name)
+
+
 @router.get(
     "/{node_id}/adapters/{adapter_number}/ports/{port_number}/capture/stream",
     dependencies=[Depends(compute_authentication)]

gns3server/api/routes/compute/qemu_nodes.py

@@ -30,6 +30,7 @@ from gns3server import schemas
 from gns3server.compute import qemu
 from gns3server.compute.qemu import Qemu
 from gns3server.compute.qemu.qemu_vm import QemuVM
+from gns3server.compute.marker.marker_manager import MarkerManager
 
 from .dependencies.authentication import compute_authentication, ws_compute_authentication
 
@@ -382,6 +383,52 @@ async def stop_qemu_node_capture(
     await node.stop_capture(adapter_number)
 
 
+@router.post(
+    "/{node_id}/adapters/{adapter_number}/ports/{port_number}/markers/start",
+    dependencies=[Depends(compute_authentication)]
+)
+async def start_qemu_node_marker(
+    *,
+    project_id: UUID,
+    adapter_number: int,
+    marker_data: schemas.MarkerCreate,
+    port_number: int = Path(..., ge=0, le=0),
+    node: QemuVM = Depends(dep_node)
+) -> dict:
+    """
+    Attach a traffic-insight ``mark`` filter to the QEMU node's uBridge bridge.
+    """
+
+    pcap_path = os.path.join(
+        node.project.markers_working_directory(),
+        f"{node.id}_{marker_data.link_id}_{marker_data.name}.pcap"
+    )
+    await node.start_marker(adapter_number, marker_data.name, marker_data.bpf, pcap_path, marker_data.tag)
+    MarkerManager.instance().register(
+        str(project_id), node.id, marker_data.name, marker_data.link_id, marker_data.tag
+    )
+    return {"pcap_file_path": str(pcap_path)}
+
+
+@router.post(
+    "/{node_id}/adapters/{adapter_number}/ports/{port_number}/markers/stop",
+    status_code=status.HTTP_204_NO_CONTENT,
+    dependencies=[Depends(compute_authentication)]
+)
+async def stop_qemu_node_marker(
+    adapter_number: int,
+    marker_data: schemas.MarkerDelete,
+    port_number: int = Path(..., ge=0, le=0),
+    node: QemuVM = Depends(dep_node)
+) -> None:
+    """
+    Remove a traffic-insight ``mark`` filter from the QEMU node's uBridge bridge.
+    """
+
+    await node.stop_marker(adapter_number, marker_data.name)
+    MarkerManager.instance().unregister(node.id, marker_data.name)
+
+
 @router.get(
     "/{node_id}/adapters/{adapter_number}/ports/{port_number}/capture/stream",
     dependencies=[Depends(compute_authentication)]

gns3server/api/routes/compute/vpcs_nodes.py

@@ -29,6 +29,7 @@ from uuid import UUID
 from gns3server import schemas
 from gns3server.compute.vpcs import VPCS
 from gns3server.compute.vpcs.vpcs_vm import VPCSVM
+from gns3server.compute.marker.marker_manager import MarkerManager
 
 from .dependencies.authentication import compute_authentication, ws_compute_authentication
 
@@ -303,6 +304,54 @@ async def stop_vpcs_node_capture(
     await node.stop_capture(port_number)
 
 
+@router.post(
+    "/{node_id}/adapters/{adapter_number}/ports/{port_number}/markers/start",
+    dependencies=[Depends(compute_authentication)]
+)
+async def start_vpcs_node_marker(
+    *,
+    project_id: UUID,
+    port_number: int,
+    marker_data: schemas.MarkerCreate,
+    adapter_number: int = Path(..., ge=0, le=0),
+    node: VPCSVM = Depends(dep_node)
+) -> dict:
+    """
+    Attach a traffic-insight ``mark`` filter to the VPCS node's uBridge bridge.
+    On BPF match uBridge emits a MARK signal and appends the packet to the pcap.
+    """
+
+    pcap_path = os.path.join(
+        node.project.markers_working_directory(),
+        f"{node.id}_{marker_data.link_id}_{marker_data.name}.pcap"
+    )
+    await node.start_marker(port_number, marker_data.name, marker_data.bpf, pcap_path, marker_data.tag)
+    MarkerManager.instance().register(
+        str(project_id), node.id, marker_data.name, marker_data.link_id, marker_data.tag
+    )
+    return {"pcap_file_path": pcap_path}
+
+
+@router.post(
+    "/{node_id}/adapters/{adapter_number}/ports/{port_number}/markers/stop",
+    status_code=status.HTTP_204_NO_CONTENT,
+    dependencies=[Depends(compute_authentication)]
+)
+async def stop_vpcs_node_marker(
+    *,
+    port_number: int,
+    marker_data: schemas.MarkerDelete,
+    adapter_number: int = Path(..., ge=0, le=0),
+    node: VPCSVM = Depends(dep_node)
+) -> None:
+    """
+    Remove a traffic-insight ``mark`` filter from the VPCS node's uBridge bridge.
+    """
+
+    await node.stop_marker(port_number, marker_data.name)
+    MarkerManager.instance().unregister(node.id, marker_data.name)
+
+
 @router.get(
     "/{node_id}/adapters/{adapter_number}/ports/{port_number}/capture/stream",
     dependencies=[Depends(compute_authentication)]