refactor: 拆分 gateway 单体为独立 crate，新增 systemd 部署方案

将 gateway 内部的 model-fetch、provider-transport、scheduler-core、
usage-runtime、video-tasks-core 模块提取为独立 crate；重构 gateway
内部模块结构（state/router/cache/data/query 等）；移除大量遗留模块
文件；新增 systemd 二进制部署骨架及相关文档；更新前端 usage 相关
API 和组件。

apps/aether-gateway/src/tests/ai_execute/stream/mod.rs

@@ -0,0 +1,26 @@
+use std::convert::Infallible;
+use std::sync::{Arc, Mutex};
+
+use axum::body::{to_bytes, Body, Bytes};
+use axum::response::Response;
+use axum::routing::any;
+use axum::{extract::Request, Json, Router};
+use http::header::{HeaderName, HeaderValue};
+use http::StatusCode;
+use serde_json::json;
+
+use crate::constants::{
+    CONTROL_EXECUTED_HEADER, CONTROL_EXECUTE_FALLBACK_HEADER, DEPENDENCY_REASON_HEADER,
+    EXECUTION_PATH_EXECUTION_RUNTIME_STREAM, EXECUTION_PATH_EXECUTION_RUNTIME_SYNC,
+    EXECUTION_PATH_HEADER, EXECUTION_PATH_LOCAL_EXECUTION_RUNTIME_MISS,
+    LOCAL_EXECUTION_RUNTIME_MISS_REASON_HEADER, TRACE_ID_HEADER,
+};
+
+use super::{
+    build_router, build_router_with_execution_runtime_override, build_router_with_state,
+    build_state_with_execution_runtime_override, start_server, wait_until, AppState,
+    FrontdoorCorsConfig, FrontdoorUserRpmConfig, GatewayFallbackMetricKind, GatewayFallbackReason,
+    UsageRuntimeConfig, VideoTaskTruthSourceMode,
+};
+
+mod decision;