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design and have been superseded by the NIO flow.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/base_node.py

@@ -1101,19 +1101,6 @@ class BaseNode:
         # bad expression must surface instead of being silently dropped.
         await self._ubridge_send(cmd)
 
-    async def _ubridge_delete_marker_filter(self, bridge_name, name):
-        """
-        Remove a `mark` filter from a uBridge bridge.
-
-        uBridge closes and flushes the filter's pcap on delete; the file itself
-        persists on disk for later replay.
-
-        :param bridge_name: uBridge bridge the filter is attached to
-        :param name: filter name previously passed to _ubridge_add_marker_filter
-        """
-
-        await self._ubridge_send(f"bridge delete_packet_filter {bridge_name} {name}")
-
     async def _ubridge_apply_markers(self, bridge_name, nio):
         """
         (Re-)apply every traffic-insight marker carried by *nio* to the uBridge
@@ -1137,7 +1124,18 @@ class BaseNode:
             pcap_path = os.path.join(
                 markers_dir, f"{self._id}_{link_id}_{name}.pcap"
             )
-            await self._ubridge_add_marker_filter(bridge_name, name, bpf, pcap_path, tag)
+            try:
+                await self._ubridge_add_marker_filter(bridge_name, name, bpf, pcap_path, tag)
+            except UbridgeError as e:
+                # Swallow BPF compile errors (warn + skip) so a single bad
+                # expression can't break link creation / node restart — mirrors
+                # _ubridge_apply_filters, which does the same for packet filters.
+                if "syntax error" in str(e).lower() or "compile filter" in str(e).lower():
+                    message = f"Warning: ignoring marker '{name}' due to BPF syntax error: {e}"
+                    log.warning(message)
+                    self.project.emit("log.warning", {"message": message})
+                    continue
+                raise
             manager.register(
                 str(self.project.id), self._id, name, link_id, tag
             )