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.rs

@@ -1,7 +1,7 @@
-use super::*;
+use crate::gateway::{AppState, GatewayError, GatewayPublicRequestContext};
+use axum::{body::Body, response::Response};
 
-const ADMIN_USERS_RUST_BACKEND_DETAIL: &str =
-    "Admin user management routes require Rust maintenance backend";
+const ADMIN_USERS_DATA_UNAVAILABLE_DETAIL: &str = "Admin user management data unavailable";
 
 #[path = "users/api_keys.rs"]
 mod users_api_keys;
@@ -35,7 +35,7 @@ use self::users_sessions::{
 };
 use self::users_shared::{
     admin_default_user_initial_gift, build_admin_users_bad_request_response,
-    build_admin_users_maintenance_response, build_admin_users_read_only_response,
+    build_admin_users_data_unavailable_response, build_admin_users_read_only_response,
     format_optional_datetime_iso8601, normalize_admin_optional_user_email,
     normalize_admin_user_role, normalize_admin_username, validate_admin_user_password,
     AdminCreateUserApiKeyRequest, AdminCreateUserRequest, AdminToggleUserApiKeyLockRequest,