feat: add Astraflow provider support

[ucloudnb666]

Summary

Adds Astraflow (by UCloud / 优刻得) as a supported LLM provider.

Astraflow is an OpenAI-compatible AI model aggregation platform supporting 200+ models, available via a global endpoint and a China endpoint.

Changes

• config.yaml — adds astraflow and astraflow-cn to the llm.provider comment so users know these are valid choices
• .env.example — documents ASTRAFLOW_API_KEY (global) and ASTRAFLOW_CN_API_KEY (China) with sign-up links
• lib/providers/__init__.py — implements get_provider_config() and is_openai_compatible() helpers that route all supported providers (including the new Astraflow entries) to their correct base_url / API-key env var

Provider details

Variant	Env var	Base URL
astraflow (global)	ASTRAFLOW_API_KEY	https://api-us-ca.umodelverse.ai/v1
astraflow-cn (China)	ASTRAFLOW_CN_API_KEY	https://api.modelverse.cn/v1

• Sign up (global): https://astraflow.ucloud-global.com
• Sign up (China): https://astraflow.ucloud.cn

Because Astraflow is fully OpenAI-compatible, no new SDK is required — only base_url + api_key need to be set, exactly like OpenRouter or Mistral.