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/maintenance/runtime/tests.rs

@@ -276,7 +276,7 @@ fn usage_cleanup_window_uses_non_overlapping_ranges() {
 #[tokio::test]
 async fn summarize_postgres_pool_uses_busy_connections_for_usage_rate() {
     let data = GatewayDataState::from_config(
-        crate::gateway::gateway_data::GatewayDataConfig::from_postgres_config(
+        crate::data::GatewayDataConfig::from_postgres_config(
             aether_data::postgres::PostgresPoolConfig {
                 database_url: "postgres://localhost/aether".to_string(),
                 min_connections: 1,