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。

crates/aether-model-fetch/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "aether-model-fetch"
+version = "0.1.0"
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+
+[dependencies]
+aether-contracts.workspace = true
+aether-data.workspace = true
+aether-provider-transport.workspace = true
+aether-scheduler-core.workspace = true
+async-trait.workspace = true
+regex.workspace = true
+serde_json.workspace = true
+uuid.workspace = true
+
+[dev-dependencies]
+tokio.workspace = true