Add SQLite database for Telegram bot user/group settings

- Replace Redis/in-memory storage with SQLite for persistence
- Add database.ts service with tables for users, groups, purchases, participants
- Update state.ts and groupState.ts to use SQLite backend
- Fix buyer_name to use display name instead of Telegram ID
- Remove legacy reminder array handlers (now using 3-slot system)
- Add better-sqlite3 dependency, remove ioredis
- Update env.example with BOT_DATABASE_PATH option
- Add data/ directory to .gitignore for database files

telegram_bot/.gitignore

@@ -14,6 +14,12 @@ logs/
 *.log
 npm-debug.log*
 
+# Database
+data/
+*.db
+*.db-wal
+*.db-shm
+
 # IDE
 .vscode/
 .idea/