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/controller/links.py

@@ -424,6 +424,85 @@ async def web_wireshark_websocket(
             pass
 
 
+@router.get(
+    "/{link_id}/markers",
+    dependencies=[Depends(has_privilege("Link.Audit"))]
+)
+async def get_markers(link: Link = Depends(dep_link)) -> dict:
+    """
+    Return all traffic-insight markers configured on this link.
+
+    Required privilege: Link.Audit
+    """
+
+    return link.markers
+
+
+@router.post(
+    "/{link_id}/markers",
+    status_code=status.HTTP_201_CREATED,
+    dependencies=[Depends(has_privilege("Link.Modify"))]
+)
+async def create_marker(
+    marker_data: schemas.MarkerCreate,
+    link: Link = Depends(dep_link)
+) -> dict:
+    """
+    Attach a traffic-insight marker to the link.
+    On BPF match uBridge emits MARK signals and appends packets to a pcap.
+
+    Required privilege: Link.Modify
+    """
+
+    await link.start_marker(
+        name=marker_data.name or f"marker-{link.id[:8]}",
+        bpf=marker_data.bpf,
+        tag=marker_data.tag,
+    )
+    return link.markers.get(marker_data.name, {})
+
+
+@router.delete(
+    "/{link_id}/markers/{marker_name}",
+    status_code=status.HTTP_204_NO_CONTENT,
+    dependencies=[Depends(has_privilege("Link.Modify"))]
+)
+async def delete_marker(
+    marker_name: str,
+    link: Link = Depends(dep_link)
+) -> None:
+    """
+    Remove a traffic-insight marker from the link.
+
+    Required privilege: Link.Modify
+    """
+
+    await link.stop_marker(marker_name)
+
+
+@router.put(
+    "/{link_id}/markers/{marker_name}",
+    dependencies=[Depends(has_privilege("Link.Modify"))]
+)
+async def update_marker(
+    marker_name: str,
+    marker_data: schemas.MarkerCreate,
+    link: Link = Depends(dep_link)
+) -> dict:
+    """
+    Update a traffic-insight marker (change BPF, tag, or enabled).
+
+    Required privilege: Link.Modify
+    """
+
+    await link.update_marker(
+        name=marker_name,
+        bpf=marker_data.bpf if marker_data.bpf else None,
+        tag=marker_data.tag,
+    )
+    return link.markers.get(marker_name, {})
+
+
 @router.get(
     "/{link_id}/iface",
     response_model=Union[schemas.UDPPortInfo, schemas.EthernetPortInfo],