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/controller/link.py

@@ -122,6 +122,7 @@ class Link:
             bpf=marker_def["bpf"],
             tag=marker_def.get("tag"),
             color=marker_def.get("color"),
+            highlight_duration=marker_def.get("highlight_duration"),
             inherited_from=def_name,
         )