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/usage/http.rs

@@ -6,7 +6,9 @@ use axum::Json;
 use serde::Deserialize;
 use serde_json::json;
 
-use crate::gateway::AppState;
+use crate::AppState;
+use aether_data::repository::audit::RequestAuditBundle;
+use aether_data::repository::usage::StoredRequestUsageAudit;
 
 #[derive(Debug, Deserialize)]
 pub(crate) struct GetRequestAuditBundleQuery {
@@ -16,7 +18,7 @@ pub(crate) struct GetRequestAuditBundleQuery {
 pub(crate) async fn get_request_usage_audit(
     State(state): State<AppState>,
     Path(request_id): Path<String>,
-) -> Result<Json<super::RequestUsageAudit>, axum::response::Response> {
+) -> Result<Json<StoredRequestUsageAudit>, axum::response::Response> {
     let usage = state
         .read_request_usage_audit(&request_id)
         .await
@@ -40,7 +42,7 @@ pub(crate) async fn get_request_audit_bundle(
     State(state): State<AppState>,
     Path(request_id): Path<String>,
     Query(query): Query<GetRequestAuditBundleQuery>,
-) -> Result<Json<super::RequestAuditBundle>, axum::response::Response> {
+) -> Result<Json<RequestAuditBundle>, axum::response::Response> {
     let attempted_only = query.attempted_only.unwrap_or(false);
     let bundle = state
         .read_request_audit_bundle(&request_id, attempted_only, current_unix_secs())