feat: add Astraflow provider support

[ucloudnb666]

Summary

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

Astraflow is an OpenAI-compatible AI model aggregation platform supporting 200+ models. Because it is fully OpenAI SDK–compatible, the integration only requires adding the base_url and api_key_url — no new SDK or subpackage needed.

Changes

pixelle_video/llm_presets.py

• Added "Astraflow" preset (global endpoint: https://api-us-ca.umodelverse.ai/v1)
• Added "Astraflow CN" preset (China endpoint: https://api.modelverse.cn/v1)

config.example.yaml

• Added comment examples for both Astraflow global and China endpoints under the LLM presets section

Provider Details

Field	Global	China
base_url	https://api-us-ca.umodelverse.ai/v1	https://api.modelverse.cn/v1
API key signup	https://astraflow.ucloud-global.com	https://astraflow.ucloud.cn
Env var	ASTRAFLOW_API_KEY	ASTRAFLOW_CN_API_KEY

Both endpoints follow the same OpenAI SDK protocol already used by every other preset in this project.

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