头像上传链路完成

头像上传流程现已具备端到端可用能力，可直接用于生产环境。

更新内容

• 实现基于签名 URL 的头像上传主流程。
• 增加并重构真实 API 测试，覆盖 URL 校验与错误处理场景。
• 优化测试初始化与动态路径生成，提升测试稳定性。
• 补充存储相关环境变量与上传链路文档说明。

关联提交

• dc08c88 feat: implement avatar upload functionality with signed URL generation
• 0f69648 feat(api): implement avatar upload with signed URL and enhance test coverage
• f866e80 test: refactor avatar upload real API tests for improved structure and functionality
• 1f27df3 test: enhance avatar upload URL validation in real API tests
• 3349f8f test: bootstrap environment for avatar upload tests and improve error messaging
• 07c1a48 refactor: enhance avatar upload tests with dynamic path generation
• f5170d8 docs: enhance documentation for storage environment variable checks and avatar upload URL generation