feat(marker): add optional color field to marker data model

Add a user-chosen hex color (e.g. '#ff5722') to each marker, stored in
Link._markers alongside bpf/tag/enabled. The color:

- Is serialized with the link (asdict) and persisted in the topology, so
  it survives project reload and is consistent across devices.
- Never reaches uBridge (compute ignores it) — it is purely a Web UI
  concern.
- Can be updated independently of the BPF (color-only changes skip the
  ubridge round-trip).

MarkerCreate schema gains color: Optional[str]; start_marker and
update_marker carry it through; REST create/update endpoints pass it.

The marker.match WebSocket event stays lean (no color) — the Web UI maps
filter -> color from the link state it already holds.

gns3server/api/routes/controller/links.py

@@ -454,12 +454,14 @@ async def create_marker(
     Required privilege: Link.Modify
     """
 
+    name = marker_data.name or f"marker-{link.id[:8]}"
     await link.start_marker(
-        name=marker_data.name or f"marker-{link.id[:8]}",
+        name=name,
         bpf=marker_data.bpf,
         tag=marker_data.tag,
+        color=marker_data.color,
     )
-    return link.markers.get(marker_data.name, {})
+    return link.markers.get(name, {})
 
 
 @router.delete(
@@ -499,6 +501,7 @@ async def update_marker(
         name=marker_name,
         bpf=marker_data.bpf if marker_data.bpf else None,
         tag=marker_data.tag,
+        color=marker_data.color,
     )
     return link.markers.get(marker_name, {})