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/Cargo.toml

@@ -13,7 +13,12 @@ aether-contracts.workspace = true
 aether-crypto.workspace = true
 aether-data.workspace = true
 aether-http.workspace = true
+aether-model-fetch.workspace = true
+aether-provider-transport.workspace = true
+aether-scheduler-core.workspace = true
 aether-runtime.workspace = true
+aether-usage-runtime.workspace = true
+aether-video-tasks-core.workspace = true
 aether-wallet.workspace = true
 async-stream.workspace = true
 async-trait.workspace = true
@@ -48,3 +53,6 @@ tracing.workspace = true
 url.workspace = true
 uuid.workspace = true
 webpki-roots.workspace = true
+
+[dev-dependencies]
+tracing-subscriber.workspace = true