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/control/route/internal.rs

@@ -18,11 +18,11 @@ pub(super) fn classify_internal_route(
             "/api/internal/gateway/finalize-sync" => "finalize_sync",
             "/api/internal/gateway/execute-sync" => "execute_sync",
             "/api/internal/gateway/execute-stream" => "execute_stream",
-            _ => "legacy_gateway",
+            _ => "unhandled",
         };
         Some(classified(
             "internal_proxy",
-            "gateway_legacy",
+            "internal_gateway",
             route_kind,
             "",
             false,