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/public/support/models/route.rs

@@ -1,3 +1,7 @@
+use std::time::{SystemTime, UNIX_EPOCH};
+
+use axum::{body::Body, response::Response};
+
 use super::models_responses::{
     build_claude_model_detail_response, build_claude_models_list_response,
     build_empty_models_list_response, build_gemini_model_detail_response,
@@ -6,7 +10,7 @@ use super::models_responses::{
     build_openai_models_list_response,
 };
 use super::models_shared::{filter_rows_for_models, models_api_format, models_detail_id};
-use super::*;
+use super::{query_param_value, AppState, GatewayPublicRequestContext};
 
 pub(super) async fn maybe_build_local_models_route_response(
     state: &AppState,