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/provider_models/available_source.rs

@@ -1,6 +1,8 @@
 use super::super::build_admin_provider_available_source_models_payload;
-use crate::gateway::handlers::admin_provider_available_source_models_path;
-use crate::gateway::{AppState, GatewayControlDecision, GatewayError, GatewayPublicRequestContext};
+use crate::control::GatewayControlDecision;
+use crate::control::GatewayPublicRequestContext;
+use crate::handlers::admin_provider_available_source_models_path;
+use crate::{AppState, GatewayError};
 use axum::{
     body::{Body, Bytes},
     http,