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/router.rs

@@ -0,0 +1,56 @@
+use axum::routing::any;
+use axum::Router;
+
+use aether_runtime::{prometheus_response, ConcurrencyError, DistributedConcurrencyError};
+
+use super::{api, handlers::proxy::proxy_request, middleware, state::AppState};
+
+pub fn build_router() -> Result<Router, reqwest::Error> {
+    Ok(build_router_with_state(AppState::new()?))
+}
+
+pub fn build_router_with_state(state: AppState) -> Router {
+    let cors_state = state.clone();
+    let mut router = Router::<AppState>::new();
+    router = api::mount_core_routes(router);
+    router = api::mount_operational_routes(router);
+    router = api::mount_ai_routes(router);
+    router = api::mount_public_support_routes(router);
+    router = api::mount_oauth_routes(router);
+    router = api::mount_internal_routes(router);
+    router = api::mount_admin_routes(router);
+    let mut router = router
+        .route("/{*path}", any(proxy_request))
+        .layer(axum::middleware::from_fn(middleware::access_log_middleware))
+        .with_state(state);
+    if cors_state.frontdoor_cors().is_some() {
+        router = router.layer(axum::middleware::from_fn_with_state(
+            cors_state,
+            middleware::frontdoor_cors_middleware,
+        ));
+    }
+    router
+}
+
+pub(crate) async fn metrics(
+    axum::extract::State(state): axum::extract::State<AppState>,
+) -> impl axum::response::IntoResponse {
+    prometheus_response(&state.metric_samples().await)
+}
+
+#[derive(Debug)]
+pub(crate) enum RequestAdmissionError {
+    Local(ConcurrencyError),
+    Distributed(DistributedConcurrencyError),
+}
+
+pub async fn serve_tcp(bind: &str) -> Result<(), Box<dyn std::error::Error>> {
+    let listener = tokio::net::TcpListener::bind(bind).await?;
+    let router = build_router()?;
+    axum::serve(
+        listener,
+        router.into_make_service_with_connect_info::<std::net::SocketAddr>(),
+    )
+    .await?;
+    Ok(())
+}