feat: add Atlas Cloud LLM preset

[binyangzhu000-sudo]

Summary

• add Atlas Cloud to the existing OpenAI-compatible LLM preset registry
• configure the default API base, model, and API-key console URL
• cover preset lookup and reverse matching with a focused test

Validation

• uv run --python 3.11 --with pytest pytest -q tests/test_llm_presets.py
• uv run --python 3.11 python -m compileall -q pixelle_video/llm_presets.py tests/test_llm_presets.py
• uvx ruff check tests/test_llm_presets.py
• uvx ruff check --ignore UP006,UP035,I001 pixelle_video/llm_presets.py
• Atlas Cloud public model catalog returned HTTP 200 and includes deepseek-ai/deepseek-v4-pro
• Atlas Cloud chat completion probe returned HTTP 200 with non-empty content

No README or promotional content changes.

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