完善客户端发布渠道并接入模板库灰度

区分发布渠道与系统，支持 dev、release 和自定义渠道
允许网站通过服务端配置选择客户端下载检测渠道
接入模板库灰度权限并阻断退出和切号后的异步操作
补齐发布、下载、灰度与会话竞态测试及当前规范

server-rs/crates/api-server/src/modules/client_downloads.rs

@@ -1,5 +1,6 @@
 use axum::{
     Json, Router,
+    extract::State,
     http::{HeaderValue, StatusCode, header::CACHE_CONTROL},
     response::{IntoResponse, Response},
     routing::get,
@@ -18,8 +19,8 @@ pub fn router(_state: AppState) -> Router<AppState> {
     Router::new().route("/api/client-downloads", get(client_downloads))
 }
 
-async fn client_downloads() -> Response {
-    download_response(fetch_latest_client_downloads().await)
+async fn client_downloads(State(state): State<AppState>) -> Response {
+    download_response(fetch_latest_client_downloads(&state.config.client_download_channel).await)
 }
 
 fn download_response(result: Result<ClientDownloads, ClientDownloadError>) -> Response {
@@ -69,6 +70,29 @@ mod tests {
     use platform_oss::client_downloads::ClientDownload as SourceDownload;
     use serde_json::{Value, json};
 
+    #[tokio::test]
+    async fn client_downloads_uses_server_config_and_does_not_accept_query_overrides() {
+        use axum::{body::Body, http::Request};
+        use tower::ServiceExt;
+        let state = AppState::new(crate::config::AppConfig {
+            client_download_channel: "dev-win".to_string(),
+            ..Default::default()
+        })
+        .unwrap();
+        let app = router(state.clone()).with_state(state);
+        let response = app
+            .oneshot(
+                Request::builder()
+                    .uri("/api/client-downloads?channel=dev&url=https://example.com")
+                    .body(Body::empty())
+                    .unwrap(),
+            )
+            .await
+            .unwrap();
+        assert_eq!(response.status(), StatusCode::BAD_GATEWAY);
+        assert_eq!(response.headers()[CACHE_CONTROL], "no-store");
+    }
+
     async fn read_success(result: ClientDownloads) -> Value {
         let response = download_response(Ok(result));
         assert_eq!(response.status(), StatusCode::OK);
@@ -151,6 +175,7 @@ mod tests {
     #[tokio::test]
     async fn upstream_failures_share_a_safe_retryable_uncached_error() {
         for error in [
+            ClientDownloadError::InvalidChannel,
             ClientDownloadError::Transport,
             ClientDownloadError::UpstreamStatus,
             ClientDownloadError::ManifestTooLarge,