feat: add Astraflow provider support

[ucloudnb666]

Summary

Adds Astraflow as a new OpenAI-compatible AI provider alongside the existing Gemini integration.

Astraflow (by UCloud / 优刻得) is an OpenAI-compatible AI model aggregation platform supporting 200+ models. Because it speaks the OpenAI REST protocol, the integration requires only setting base_url + api_key — no new SDK is needed.

Changes

File	What changed
config.conf.example	New [Astraflow] config section documenting the two endpoints and their env vars
src/app/config.py	Default config values for the [Astraflow] section
src/app/services/astraflow_client.py	Thin async client wrapping the Astraflow REST API via httpx
src/app/endpoints/astraflow.py	FastAPI router exposing /astraflow/v1/models and /astraflow/v1/chat/completions
src/app/main.py	Import + register the new Astraflow router

Configuration

Copy config.conf.example to config.conf and fill in your key:

[Astraflow]
# Global endpoint
api_key = sk-...          # or set ASTRAFLOW_API_KEY env var
# China endpoint (optional)
cn_api_key =              # or set ASTRAFLOW_CN_API_KEY env var
use_cn_endpoint = false

Endpoints

• GET /astraflow/v1/models — list available models
• POST /astraflow/v1/chat/completions — OpenAI-compatible chat completions (streaming supported)

Links

• Global: https://astraflow.ucloud-global.com
• China: https://astraflow.ucloud.cn