feat: implement async database session management and CBSL currency exchange rate service with persistent caching

Simplify Telegram Bot to unified daily rate list subscription and drop legacy tables

chore: handle startup db failures gracefully and add long polling fallback

feat: implement aiogram command handlers and interactive currency keyboards for Telegram bot

Fix markdown parsing, asyncpg prepared statements, and flexible command arguments

feat: implement database schema, async session management, and bot command handlers for FlyRates

first commit