fix(api): register user_management router in feature_routers.py

api/user_management module existed but was never registered — endpoints
were unreachable. Ported registration from main (#4571) that bypassed
Dev_new_gui.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: mrveiss

autobot-backend/initialization/router_registry/feature_routers.py

@@ -461,6 +461,13 @@
         ["resilience", "monitoring"],
         "error_resilience",
     ),
+    # User management (users, teams, organizations) — router defines /user-management internally
+    (
+        "api.user_management",
+        "",
+        ["user-management", "users"],
+        "user_management",
+    ),
     # Issue #1803: Plugin manager endpoints (list, discover, load/unload/enable/disable, config)
     ("plugin_manager", "", ["plugins"], "plugin_manager"),
     # Issue #1803: Plugin and agent marketplace — community catalog