feat(saas): add trusted_proxies config for reverse proxy rate limiting

- Add trusted_proxies field to ServerConfig (Vec<String>, serde default)
- Default value is empty vector (no proxy trust until explicitly configured)
- Development config: trust localhost IPs (127.0.0.1, ::1)
- Production config: placeholder localhost IPs with comment to replace

config/saas-development.toml

@@ -5,6 +5,7 @@
 host = "0.0.0.0"
 port = 8080
 cors_origins = []  # 空 = 开发模式允许所有来源
+trusted_proxies = ["127.0.0.1", "::1"]
 
 [database]
 url = "postgres://postgres:123123@localhost:5432/zclaw"