Enhance Telegram bot functionality and improve error handling

- Introduced a new function to set the default menu button for the Telegram bot's Web App.
- Updated the initData validation process to provide detailed error messages for authorization failures.
- Refactored the validate_init_data function to return both username and reason for validation failure.
- Enhanced the web application to handle access denial more gracefully, providing users with hints on how to access the calendar.
- Improved the README with additional instructions for configuring the bot's menu button and Web App URL.
- Updated tests to reflect changes in the validation process and error handling.

db/session.py

@@ -1,4 +1,10 @@
-"""SQLAlchemy engine and session factory."""
+"""SQLAlchemy engine and session factory.
+
+Note: Engine and session factory are cached globally per process. Only one
+DATABASE_URL is effectively used for the process lifetime. Using a different
+URL later (e.g. in tests with in-memory SQLite) would still use the first
+engine. To support multiple URLs, cache by database_url (e.g. a dict keyed by URL).
+"""
 
 from contextlib import contextmanager
 from typing import Generator