feat: add i18n localization system (en/es/de) with admin panel

- Add i18n module with tForUser/tForLang/t functions and {{param}} interpolation
- Add 3 locale files: en.json, es.json, de.json (201 keys each)
- Add language selection on /start and /language command with flag emojis
- Localize all bot user-facing strings (handlers, keyboards, errors)
- Localize messageRouter keyboard matching via locale keys
- Add DB migrations 008 (language column) and 009 (language_set column)
- Add localization admin tab at /locales for editing translations
- Add userService.getUserLanguage/setUserLanguage methods
- Cache user object on msg.__user to avoid triple DB fetch
- Idempotent migrations with checkColumnExists guards
- Error boundary on i18n locale file loading
- Admin locales route uses AVAILABLE_LANGUAGES import

src/services/userService.js

@@ -1,13 +1,14 @@
 // userService.js
 
 import db from "../config/database.js";
+import config from '../config/config.js';
 import Wallet from "../models/Wallet.js";
 import WalletUtils from "../utils/walletUtils.js";
 import logger from "../utils/logger.js";
 
 const ALLOWED_USER_FIELDS = new Set([
   'telegram_id', 'username', 'country', 'city',
-  'district', 'status', 'total_balance', 'bonus_balance'
+  'district', 'status', 'total_balance', 'bonus_balance', 'language', 'language_set'
 ]);
 
 
@@ -215,6 +216,19 @@ class UserService {
             throw error;
         }
     }
+
+    static async getUserLanguage(telegramId) {
+        const user = await this.getUserByTelegramId(telegramId);
+        return user?.language || 'en';
+    }
+
+    static async setUserLanguage(telegramId, lang) {
+        const normalizedTelegramId = this.normalizeTelegramId(telegramId);
+        await db.runAsync(
+            'UPDATE users SET language = ?, language_set = 1 WHERE telegram_id = ?',
+            [lang, normalizedTelegramId]
+        );
+    }
 }
 
 export default UserService;