refactor(marker): converge to filter single-path model, remove dual-apply endpoints

Markers now follow exactly the same apply pattern as packet filters:
state lives in Link._markers, application goes through NIO
(update() -> PUT /nio -> _ubridge_apply_markers). The former
immediate-apply REST endpoints (/markers/start, /markers/stop on
the compute side) and the per-node start_marker/stop_marker methods
are removed — they were a legacy of the original capture-inspired
design and have been superseded by the NIO flow.

Changes:
- controller/udp_link: start_marker/stop_marker/update_marker now
  set _markers state + call self.update() (mirrors update_filters).
  Removed _marker_capture_nodes runtime dict and its helpers.
- controller/project: _create_link_from_topology_data restores
  _markers directly from persisted data (with BPF validation,
  like filter reload). No long calls start_marker during load.
- compute: _ubridge_apply_markers swallows BPF compile errors
  (warn+skip), matching _ubridge_apply_filters behaviour so a
  single bad expression cannot break link creation / node restart.
- Removed: /markers/start,stop endpoints (6 handlers across
  vpcs/qemu/docker route files), node start_marker/stop_marker
  methods (3 VM files), _ubridge_delete_marker_filter,
  _marker_capture_nodes, MarkerDelete schema.

Net: ~280 lines of dead code removed; marker and packet filter now
share a single, unified apply path via the NIO.

gns3server/compute/docker/docker_vm.py

@@ -1440,44 +1440,6 @@ class DockerVM(BaseNode):
             )
         )
 
-    async def start_marker(self, adapter_number, name, bpf, pcap_path, tag=None):
-        """
-        Attach a traffic-insight ``mark`` filter to this adapter's uBridge bridge.
-        On BPF match uBridge emits a MARK signal and appends the packet to the pcap.
-
-        :param adapter_number: adapter number
-        :param name: stable filter name — pcap identity and echoed in MARK signals
-        :param bpf: libpcap BPF expression
-        :param pcap_path: absolute path ubridge appends matched packets to
-        :param tag: optional correlation id echoed in MARK signals
-        """
-
-        if self.status == "started" and self.ubridge:
-            adapter = f"bridge{adapter_number}"
-            await self._ubridge_add_marker_filter(adapter, name, bpf, pcap_path, tag)
-        log.info(
-            "Docker VM '{name}' [{id}]: starting marker '{marker}' on adapter {adapter_number}".format(
-                name=self.name, id=self.id, marker=name, adapter_number=adapter_number
-            )
-        )
-
-    async def stop_marker(self, adapter_number, name):
-        """
-        Remove a traffic-insight ``mark`` filter from this adapter's uBridge bridge.
-
-        :param adapter_number: adapter number
-        :param name: filter name previously passed to start_marker
-        """
-
-        if self.status == "started" and self.ubridge:
-            adapter = f"bridge{adapter_number}"
-            await self._ubridge_delete_marker_filter(adapter, name)
-        log.info(
-            "Docker VM '{name}' [{id}]: stopping marker '{marker}' on adapter {adapter_number}".format(
-                name=self.name, id=self.id, marker=name, adapter_number=adapter_number
-            )
-        )
-
     async def _get_log(self):
         """
         Returns the log from the container