Upgrade InsightFace models to antelopev2

[guesswh0]

• Refactor RetinaFaceDetector and ArcFaceEmbedder to use new base classes InsightFaceDetector and InsightFaceEmbedder.
• Implement SCRFDDetector (SCRFD) and ArcFaceResNet100Embedder (ResNet100) using the antelopev2 model pack.
• Add antelopev2 download logic using a Hugging Face mirror.
• Add robust path resolution to handle varying zip extraction structures (nested vs flat).
• Add tests/test_insightface_models_mock.py to verify model initialization and fallback logic in restricted environments.

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PR created automatically by Jules for task 11739023599004647660 started by @guesswh0

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