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/src/config/index.ts

@@ -31,8 +31,8 @@ export const config = {
   frontend: {
     baseUrl: optional('FRONTEND_BASE_URL', 'http://localhost:3001'),
   },
-  redis: {
-    url: process.env.REDIS_URL || null,
+  database: {
+    path: process.env.BOT_DATABASE_PATH || null, // Defaults to ./data/bot.db in database.ts
   },
   bot: {
     maxTicketsPerPurchase: optionalInt('MAX_TICKETS_PER_PURCHASE', 100),