Improve health check and UI messaging for fallback mode

Improve fallback colorization with better color hints and saturation

Add simple colorization fallback using LAB color space when model fails to load

Fix image colorization: Add PyTorch GAN colorizer fallback, update Dockerfile to use main_fastai, and add missing dependencies

Integrate FastAI colorization with Firebase auth and Gradio UI - Replace main.py with FastAI implementation - Add Gradio interface for Space UI - Add Firebase authentication to /colorize endpoint - Add curl examples documentation - Update test.py with User-Agent headers