feat: add Astraflow provider support

[ucloudnb666]

Adds Astraflow (by UCloud) as an OpenAI-compatible provider, mirroring the existing OpenRouter integration pattern.

Changes:

• config/config.example.yml — new astraflow_api_key and astraflow_api_base fields (global + China endpoints documented)
• bot/config.py — reads the two new config keys with safe defaults
• bot/openai_utils.py — initialises an astraflow_client when the key is present; _get_client_for_model routes models with provider: astraflow to it
• config/models.yml — adds two representative Astraflow models (astraflow/gpt-4o and astraflow/deepseek-r1) to show the pattern; users can add any of the 200+ supported models the same way

Usage: set astraflow_api_key in config/config.yml and add models with provider: astraflow to config/models.yml.