feat: add Astraflow provider support

[ucloudnb666]

Summary

Adds Astraflow as a supported AI provider.

Astraflow (by UCloud / 优刻得) is an OpenAI-compatible AI model aggregation platform supporting 200+ models. Because it is fully OpenAI-compatible, this integration reuses the existing gpt package — the only change needed is setting the base_url to Astraflow's endpoint and reading the API key from the appropriate environment variable.

Two endpoints are supported:

Variant	Endpoint	Env var
Global	https://api-us-ca.umodelverse.ai/v1	ASTRAFLOW_API_KEY
China	https://api.modelverse.cn/v1	ASTRAFLOW_CN_API_KEY

Files changed

• backend/.env.example — documents the two new env vars and the provider mode toggle
• backend/internal/provider/factory/factory.go — registers astraflow and astraflow_cn providers using the existing gpt.New() constructor with Astraflow base URLs

Usage

# Global endpoint
KLEIN_PROVIDER_ASTRAFLOW=real
ASTRAFLOW_API_KEY=sk-...

# China endpoint
KLEIN_PROVIDER_ASTRAFLOW_CN=real
ASTRAFLOW_CN_API_KEY=sk-...