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/__init__.py

@@ -20,7 +20,7 @@ from .common import ErrorMessage
 from .version import Version
 
 # Controller schemas
-from .controller.links import LinkCreate, LinkUpdate, Link, UDPPortInfo, EthernetPortInfo, LinkCapture
+from .controller.links import LinkCreate, LinkUpdate, Link, UDPPortInfo, EthernetPortInfo, LinkCapture, MarkerCreate, MarkerDelete
 from .controller.computes import ComputeCreate, ComputeUpdate, ComputeVirtualBoxVM, ComputeVMwareVM, ComputeDockerImage, AutoIdlePC, Compute
 from .controller.templates import TemplateCreate, TemplateUpdate, TemplateUsage, Template
 from .controller.images import Image, ImageType

gns3server/schemas/config.py

@@ -153,6 +153,12 @@ class ServerSettings(BaseModel):
     udp_start_port_range: int = Field(10000, gt=0, le=65535)
     udp_end_port_range: int = Field(30000, gt=0, le=65535)
     ubridge_path: str = "ubridge"
+    # Marker (traffic-insight) UDP sink: one listener per compute process that
+    # receives ubridge MARK signals from every ubridge on this host. The host
+    # defaults to loopback because ubridge runs on the same host as the compute.
+    # port=0 lets the OS choose a free port (read back and handed to ubridge).
+    marker_listen_host: str = "127.0.0.1"
+    marker_listen_port: int = Field(0, ge=0, le=65535)
     compute_username: str = "gns3"
     compute_password: SecretStr = SecretStr("")
     allowed_interfaces: List[str] = Field(default_factory=list)

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