feat: 新增文件上传服务

- 添加 UploadHandler 处理文件上传
- 添加 UploadService 实现文件存储
- 配置上传文件大小限制和存储路径

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/config/config.yaml

@@ -4,3 +4,15 @@ jwt_secret: "dev-secret-key"
 # Docker 内访问用 db:3306，本地访问用 localhost:6036
 database_url: "root:root@tcp(localhost:6036)/x_agents?charset=utf8mb4&parseTime=True&loc=Local"
 python_service_url: "http://localhost:8081"
+
+# 文件上传配置 (local 或 minio)
+upload_mode: "local"
+upload_local_path: "resource/files"
+server_base_url: "http://localhost:8082"
+
+# MinIO 配置
+minio_endpoint: "localhost:9000"
+minio_access_key: ""
+minio_secret_key: ""
+minio_bucket: "x-agents"
+minio_use_ssl: false