GitLab

- 同步 OpenAI 图片生成与编辑接口
- 接入图片请求解析、账号调度、转发与用量记录
- 接入图片计费与图片用量落库
- 限制 OAuth 生图仅支持无显式模型和尺寸的基础请求

Introduce endpoint.go with shared constants, NormalizeInboundEndpoint, DeriveUpstreamEndpoint, InboundEndpointMiddleware, and context helpers. This replaces the two separate normalization implementations (OpenAI and Gateway) with a single source of truth. Includes comprehensive test coverage.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode

)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>