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design and have been superseded by the NIO flow.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/schemas/controller/links.py

@@ -159,9 +159,3 @@ class MarkerCreate(BaseModel):
     )
 
 
-class MarkerDelete(BaseModel):
-    """
-    Body for removing a traffic-insight marker from a link.
-    """
-
-    name: str