feat(agnes): 接入 Agnes 图像和视频模型

[Ferict]

Summary

• Add Agnes image generation client
• Add Agnes video generation client
• Add Agnes provider settings in Web UI
• Add Agnes LLM presets
• Add SOCKS proxy support via httpx[socks]
• Normalize Agnes video num_frames to the required 8n + 1 rule
• Prefer Agnes video_id result polling endpoint
• Preserve remote Agnes image URLs for image-to-video workflows
• Add Agnes image/video client tests

Verification

• uv run --extra dev pytest
• uv run python -m compileall pixelle_video web tests
• uv run --extra dev ruff check pixelle_video/services/api_services/image_agnes.py pixelle_video/services/api_services/video_agnes.py tests/services/api_services/agnes_image_test.py tests/services/api_services/agnes_video_test.py

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