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/headers.rs

@@ -0,0 +1,41 @@
+pub fn should_skip_request_header(name: &str) -> bool {
+    let normalized = name.to_ascii_lowercase();
+    matches!(
+        normalized.as_str(),
+        "connection"
+            | "keep-alive"
+            | "proxy-authenticate"
+            | "proxy-authorization"
+            | "proxy-connection"
+            | "te"
+            | "trailer"
+            | "transfer-encoding"
+            | "upgrade"
+            | "x-aether-execution-path"
+            | "x-aether-dependency-reason"
+            | "x-aether-control-execute-fallback"
+            | "x-aether-rate-limit-preflight"
+    )
+}
+
+pub fn should_skip_upstream_passthrough_header(name: &str) -> bool {
+    matches!(
+        name.to_ascii_lowercase().as_str(),
+        "authorization"
+            | "x-api-key"
+            | "x-goog-api-key"
+            | "host"
+            | "content-length"
+            | "transfer-encoding"
+            | "connection"
+            | "accept-encoding"
+            | "content-encoding"
+            | "x-real-ip"
+            | "x-real-proto"
+            | "x-forwarded-for"
+            | "x-forwarded-proto"
+            | "x-forwarded-scheme"
+            | "x-forwarded-host"
+            | "x-forwarded-port"
+    ) || should_skip_request_header(name)
+}