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-provider-transport/src/antigravity.rs

@@ -0,0 +1,23 @@
+mod auth;
+mod policy;
+mod request;
+mod url;
+
+pub use auth::{
+    build_antigravity_static_identity_headers, resolve_local_antigravity_request_auth,
+    AntigravityRequestAuth, AntigravityRequestAuthSupport, AntigravityRequestAuthUnsupportedReason,
+    ANTIGRAVITY_PROVIDER_TYPE, ANTIGRAVITY_REQUEST_USER_AGENT,
+};
+pub use policy::{
+    classify_local_antigravity_request_support, AntigravityRequestSideSpec,
+    AntigravityRequestSideSupport, AntigravityRequestSideUnsupportedReason,
+};
+pub use request::{
+    build_antigravity_safe_v1internal_request, classify_antigravity_safe_request_body,
+    AntigravityEnvelopeRequestType, AntigravityRequestEnvelopeSupport,
+    AntigravityRequestEnvelopeUnsupportedReason,
+};
+pub use url::{
+    build_antigravity_v1internal_url, AntigravityRequestUrlAction,
+    ANTIGRAVITY_V1INTERNAL_PATH_TEMPLATE,
+};