refactor: replace standard logging with a centralized loguru configuration and interception handler

refactor: update system prompts safety guidelines

refactor: add NSFW and gibbreish words filtering

refactor: changing to gemini model, modularize constants and schemas and update user daily generation limit

feat: implement daily request rate limiting with per-user usage tracking and admin bypass

feat: implement search functionality, add background task queue logic for title conflicts, and enhance auth state synchronization