Update configuration and access control for Telegram miniapp

- Added ALLOWED_USERNAMES and ADMIN_USERNAMES to .env.example for user access control.
- Implemented validation of Telegram Web App initData in a new telegram_auth.py module.
- Enhanced API to check user access before fetching duties.
- Updated README with instructions for configuring miniapp access.
- Modified .dockerignore and .gitignore to include data directory and database files.

webapp/index.html

@@ -20,7 +20,12 @@
     <div class="duty-list" id="dutyList"></div>
     <div class="loading" id="loading">Загрузка…</div>
     <div class="error" id="error" hidden></div>
+    <div class="access-denied" id="accessDenied" hidden>
+      <p>Доступ запрещён.</p>
+      <p class="muted">Откройте календарь из Telegram.</p>
+    </div>
   </div>
+  <script src="https://telegram.org/js/telegram-web-app.js"></script>
   <script src="app.js"></script>
 </body>
 </html>