refactor: 移除 Python upstream 依赖，清理全部 legacy/Python 兼容层

- 移除 upstream_base_url 参数及 AETHER_GATEWAY_UPSTREAM 环境变量，gateway 不再需要指向 Python 宿主
- 删除所有 LEGACY_*/PYTHON_* 常量、路由组、header 定义及 sunset/phaseout 机制
- 将 legacy_gateway_bridge 重命名为 internal_gateway，executor 相关命名统一为 execution_runtime
- dev.sh 新增 Postgres/Redis 预检查，移除 upstream 相关启动参数和提示
- 新增 ai_public 路由处理器
- 全量适配 handler、test、state、control 等模块的命名和接口变更

apps/aether-gateway/src/handlers/admin/users/routes.rs

@@ -1,4 +1,15 @@
-use super::*;
+use super::{
+    build_admin_create_user_api_key_response, build_admin_create_user_response,
+    build_admin_delete_user_api_key_response, build_admin_delete_user_response,
+    build_admin_delete_user_session_response, build_admin_delete_user_sessions_response,
+    build_admin_get_user_response, build_admin_list_user_api_keys_response,
+    build_admin_list_user_sessions_response, build_admin_list_users_response,
+    build_admin_reveal_user_api_key_response, build_admin_toggle_user_api_key_lock_response,
+    build_admin_update_user_api_key_response, build_admin_update_user_response,
+    build_admin_users_data_unavailable_response,
+};
+use crate::gateway::{AppState, GatewayError, GatewayPublicRequestContext};
+use axum::{body::Body, http, response::Response};
 
 fn is_admin_users_route(request_context: &GatewayPublicRequestContext) -> bool {
     let path = request_context.request_path.as_str();
@@ -110,7 +121,7 @@ pub(super) async fn maybe_build_local_admin_users_routes_response(
             if !is_admin_users_route(request_context) {
                 return Ok(None);
             }
-            Ok(Some(build_admin_users_maintenance_response()))
+            Ok(Some(build_admin_users_data_unavailable_response()))
         }
     }
 }