Enhance database initialization and improve command handling

- Added `__all__` declaration in `db/__init__.py` for better module export management.
- Simplified command text formatting in `handlers/commands.py` for improved readability.
- Refactored error handler function signature in `handlers/errors.py` for better code style.
- Introduced comprehensive tests for API duties and Telegram authentication in new test files.

handlers/commands.py

@@ -48,9 +48,7 @@ async def start(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
 async def help_cmd(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
     if update.message:
         await update.message.reply_text(
-            "Доступные команды:\n"
-            "/start — Начать\n"
-            "/help — Показать эту справку"
+            "Доступные команды:\n/start — Начать\n/help — Показать эту справку"
         )