修复AI分析SSE流断开：中间件透传Flusher、DB Key生效、错误提示不覆盖

web/backend/internal/middleware/logger.go

@@ -39,6 +39,13 @@ func (r *statusRecorder) WriteHeader(code int) {
 	r.ResponseWriter.WriteHeader(code)
 }
 
+// Flush 透传 http.Flusher,避免 SSE 流式响应被中间件阻断。
+func (r *statusRecorder) Flush() {
+	if f, ok := r.ResponseWriter.(http.Flusher); ok {
+		f.Flush()
+	}
+}
+
 func writeJSON(w http.ResponseWriter, status int, body any) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(status)