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/middleware/mod.rs

@@ -2,4 +2,4 @@ mod frontdoor_cors;
 mod strip_cf_headers;
 
 pub(crate) use frontdoor_cors::frontdoor_cors_middleware;
-pub(crate) use strip_cf_headers::strip_cf_headers_middleware;
+pub use strip_cf_headers::strip_cf_headers_middleware;

apps/aether-gateway/src/middleware/strip_cf_headers.rs

@@ -15,7 +15,7 @@ static CF_HEADERS: &[&str] = &[
     "cf-warp-tag-id",
 ];
 
-pub(crate) async fn strip_cf_headers_middleware(mut request: Request, next: Next) -> Response {
+pub async fn strip_cf_headers_middleware(mut request: Request, next: Next) -> Response {
     // Strip CF headers from the incoming request
     for name in CF_HEADERS {
         if let Ok(header) = HeaderName::from_bytes(name.as_bytes()) {