fix: background thread init so uvicorn binds port 7860 immediately on HF Spaces

fix: multi-model fallback cascade (4 models x 3 keys = 12 combos) + FastAPI root redirect

fix: lazy GeminiKeyManager init + safe CORS default

feat: full optimization - Groq LLM, disease cache, deploy configs

Push all code to huy_xau_trai repository