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/lib.rs

@@ -30,7 +30,7 @@ pub use gateway::exports::{
     build_execution_runtime_router, build_execution_runtime_router_with_request_concurrency_limit,
     build_execution_runtime_router_with_request_gates, build_router, build_router_with_state,
     build_tunnel_runtime_router_with_state, serve_execution_runtime_tcp,
-    serve_execution_runtime_unix, serve_tcp, strip_cf_headers_middleware, tunnel_protocol, AppState,
-    FrontdoorCorsConfig, FrontdoorUserRpmConfig, GatewayDataConfig, TunnelConnConfig,
+    serve_execution_runtime_unix, serve_tcp, strip_cf_headers_middleware, tunnel_protocol,
+    AppState, FrontdoorCorsConfig, FrontdoorUserRpmConfig, GatewayDataConfig, TunnelConnConfig,
     TunnelControlPlaneClient, TunnelRuntimeState, UsageRuntimeConfig, VideoTaskTruthSourceMode,
 };