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/schemas/controller/links.py

@@ -62,6 +62,10 @@ class LinkBase(BaseModel):
     suspend: Optional[bool] = None
     link_style: Optional[LinkStyle] = None
     filters: Optional[dict] = None
+    markers: Optional[dict] = Field(
+        None,
+        description="Traffic-insight markers on this link: name → {bpf, tag, enabled}"
+    )
     show_filters_icon: Optional[bool] = Field(
         True,
         description="Show filters icon in Web UI"
@@ -135,3 +139,25 @@ class LinkCapture(BaseModel):
     data_link_type: str = "DLT_EN10MB"
     capture_file_name: Optional[str] = None
     wireshark: bool = False
+
+
+class MarkerCreate(BaseModel):
+    """
+    Body for attaching a traffic-insight marker to a link.
+
+    ``name`` is optional at the controller REST layer (auto-generated when
+    absent) but always set when the controller forwards to the compute.
+    """
+
+    name: Optional[str] = None
+    bpf: str
+    tag: Optional[int] = None
+    link_id: Optional[str] = None
+
+
+class MarkerDelete(BaseModel):
+    """
+    Body for removing a traffic-insight marker from a link.
+    """
+
+    name: str