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/headers.rs

@@ -1,6 +1,6 @@
 use std::collections::BTreeMap;
 
-use crate::gateway::constants::*;
+use crate::constants::*;
 use uuid::Uuid;
 
 pub(crate) fn extract_or_generate_trace_id(headers: &http::HeaderMap) -> String {
@@ -21,45 +21,11 @@ pub(crate) fn header_value_u64(headers: &http::HeaderMap, key: &str) -> Option<u
 }
 
 pub(crate) 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"
-    )
+    crate::provider_transport::should_skip_request_header(name)
 }
 
 pub(crate) 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)
+    crate::provider_transport::should_skip_upstream_passthrough_header(name)
 }
 
 pub(crate) fn should_skip_response_header(name: &str) -> bool {