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/video_tasks/routes.rs

@@ -1,7 +1,20 @@
 use crate::gateway;
+use crate::gateway::handlers::query_param_value;
+use crate::gateway::{AppState, GatewayError, GatewayPublicRequestContext};
+use axum::{
+    body::Body,
+    http,
+    response::{IntoResponse, Response},
+    Json,
+};
+use serde_json::json;
 
-use super::video_tasks_builders::*;
-use super::*;
+use super::super::build_proxy_error_response;
+use super::video_tasks_builders::{
+    admin_video_task_detail_id_from_path, admin_video_task_nested_id_from_path,
+    admin_video_task_status_name, admin_video_task_timestamp, build_admin_video_task_list_item,
+    build_admin_video_task_provider_names, current_admin_video_task_unix_secs,
+};
 
 pub(super) async fn maybe_build_local_admin_video_tasks_response(
     state: &AppState,