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/migrations/runner.js

@@ -42,6 +42,8 @@ export async function runMigrations() {
     (await import('./005_audit_log.js')).default,
     (await import('./006_subcategories.js')).default,
     (await import('./007_commission_payments.js')).default,
+    (await import('./008_user_language.js')).default,
+    (await import('./009_user_language_set.js')).default,
   ];
 
   for (let i = currentVersion; i < migrations.length; i++) {