refactor(performance): replace all json imports with json_utils

fix: use lazy initialization for agents to avoid import-time errors

fix: resolve agent initialization error causing maintenance mode

feat(chat): implement conversational AI capabilities in Carlos Drummond agent

fix: remove agent initialization from ChatService constructor

fix: use cache_service instance instead of CacheService class

fix: remove incorrect CacheService import from chat service

fix: add detailed logging and error handling for chat API

feat(chat): implement complete conversational chat API with SSE streaming