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/handlers/admin/endpoints/routes.rs

@@ -5,11 +5,12 @@ use super::super::{
     build_admin_update_provider_endpoint_record, endpoint_key_counts_by_format,
     key_api_formats_without_entry,
 };
-use crate::gateway::api::ai::admin_default_body_rules_for_signature;
-use crate::gateway::handlers::{
+use crate::api::ai::admin_default_body_rules_for_signature;
+use crate::control::GatewayPublicRequestContext;
+use crate::handlers::{
     query_param_value, AdminProviderEndpointCreateRequest, AdminProviderEndpointUpdateRequest,
 };
-use crate::gateway::{AppState, GatewayError, GatewayPublicRequestContext};
+use crate::{AppState, GatewayError};
 use axum::{
     body::{Body, Bytes},
     http,