chore: 配置 gosec 排除规则

- 新增 backend/.gosec.yaml 配置文件，排除 G704 (SSRF) 检查
- 更新 security-scan.yml workflow，使用 gosec 配置文件
- 原因：作为 API 网关平台，需要代理请求到配置的上游服务，所有上游 URL 来自管理员配置而非用户输入

backend/.gosec.yaml

@@ -0,0 +1,7 @@
+global:
+  # Exclude G704 (SSRF via taint analysis) - this is an API gateway platform
+  # that by design proxies requests to configurable upstream services.
+  # All upstream URLs are sourced from admin-configured settings or known
+  # third-party API endpoints, not from end-user input.
+  exclude:
+    - G704