Add configuration rules, refactor settings management, and enhance import functionality

- Introduced a new configuration file `.cursorrules` to define coding standards, error handling, testing requirements, and project-specific guidelines.
- Refactored `config.py` to implement a `Settings` dataclass for better management of environment variables, improving testability and maintainability.
- Updated the import duty schedule handler to utilize session management with `session_scope`, ensuring proper database session handling.
- Enhanced the import service to streamline the duty schedule import process, improving code organization and readability.
- Added new service layer functions to encapsulate business logic related to group duty pinning and duty schedule imports.
- Updated README documentation to reflect the new configuration structure and improved import functionality.

services/__init__.py

@@ -0,0 +1,27 @@
+"""Service layer: business logic and orchestration.
+
+Services accept a DB session from the caller (handlers open session_scope and pass session).
+No Telegram or HTTP dependencies; repository handles persistence.
+"""
+
+from services.group_duty_pin_service import (
+    format_duty_message,
+    get_duty_message_text,
+    get_next_shift_end_utc,
+    save_pin,
+    delete_pin,
+    get_message_id,
+    get_all_pin_chat_ids,
+)
+from services.import_service import run_import
+
+__all__ = [
+    "format_duty_message",
+    "get_duty_message_text",
+    "get_next_shift_end_utc",
+    "save_pin",
+    "delete_pin",
+    "get_message_id",
+    "get_all_pin_chat_ids",
+    "run_import",
+]