#44 feat: enhance /api/models endpoint with model existence checking

Enhanced the /api/models endpoint to provide comprehensive model information including: - Model existence checking logic in ModelManager - Enhanced JSON response structure with ModelDetails - Architecture-specific recommended model fields - Backward compatibility maintained The endpoint now returns detailed information about required models (VAE, CLIP-L, CLIP-G, T5XXL, etc.) including whether they exist on the filesystem, their file sizes, and SHA256 hashes. Files modified: - include/model_manager.h - Added ModelDetails structure and method declarations - src/model_manager.cpp - Implemented existence checking logic - src/server.cpp - Enhanced /api/models endpoint with new JSON structure