feat: implement weekly MLOps retraining loop and specialized multi-model router

fix: manually implement protected /metrics route in main.py to fix expose type error

config: protect /metrics endpoint with Basic Auth (admin/admin) for Grafana compliance

feat: integrate Prometheus metrics suite and polish landing page navigation

feat: implement startup ONNX model downloader and live inference service

chore: initial project structure with uv config and gitignore