feat: add Telegram customers management system with admin panel

Implement comprehensive Telegram customers storage and management functionality:

Backend:
- Add database migration for telecart_customers table with indexes
- Create TelegramCustomer model with CRUD operations
- Implement TelegramCustomerService for business logic
- Add TelegramCustomerHandler for API endpoint (saveOrUpdate)
- Add TelegramCustomersHandler for admin API (getCustomers with pagination, filtering, sorting)
- Add SendMessageHandler for sending messages to customers via Telegram
- Create custom exceptions: TelegramCustomerNotFoundException, TelegramCustomerWriteNotAllowedException
- Refactor TelegramInitDataDecoder to separate decoding logic
- Add TelegramHeader enum for header constants
- Update SignatureValidator to use TelegramInitDataDecoder
- Register new routes in bastion/routes.php and src/routes.php

Frontend (Admin):
- Add CustomersView.vue component with PrimeVue DataTable
- Implement advanced filtering (text, date, boolean filters)
- Add column visibility toggle functionality
- Add global search with debounce
- Implement message sending dialog with validation
- Add Russian locale for PrimeVue components
- Add navigation link in App.vue
- Register route in router

Frontend (SPA):
- Add saveTelegramCustomer utility function
- Integrate automatic customer data saving on app initialization
- Extract user data from Telegram.WebApp.initDataUnsafe

The system automatically saves/updates customer data when users access the Telegram Mini App,
and provides admin interface for viewing, filtering, and messaging customers.

BREAKING CHANGE: None

frontend/admin/src/App.vue

@@ -34,6 +34,10 @@
         <RouterLink :to="{name: 'formbuilder'}">Форма заказа</RouterLink>
       </li>
 
+      <li :class="{active: route.name === 'customers'}">
+        <RouterLink :to="{name: 'customers'}">Telegram Покупатели</RouterLink>
+      </li>
+
       <li :class="{active: route.name === 'logs'}">
         <RouterLink :to="{name: 'logs'}">Журнал событий</RouterLink>
       </li>