feat(marker): add highlight_duration render hint

Per-marker UI hint (milliseconds, ge=1) for how long the Web UI keeps a
marker highlighted after a match. Mirrors color: stored on the link,
persisted in the topology, inherited via project-level definitions, and
never sent to uBridge. Omitted = null = frontend uses its own default.

The default is intentionally NOT set in the schema: MarkerCreate also
backs PUT updates, so a schema default would make every partial update
silently reset the value. None means "not provided" (keep existing on
update / frontend decides on create).

gns3server/api/routes/controller/links.py

@@ -464,6 +464,7 @@ async def create_marker(
         bpf=marker_data.bpf,
         tag=marker_data.tag,
         color=marker_data.color,
+        highlight_duration=marker_data.highlight_duration,
     )
     return link.markers.get(name, {})
 
@@ -507,6 +508,7 @@ async def update_marker(
         tag=marker_data.tag,
         color=marker_data.color,
         enabled=marker_data.enabled,
+        highlight_duration=marker_data.highlight_duration,
     )
     return link.markers.get(marker_name, {})