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.

utils/__init__.py

@@ -0,0 +1,24 @@
+"""Shared utilities: date/ISO helpers, user display names, handover time parsing.
+
+Used by handlers, API, and services. No DB or Telegram dependencies.
+"""
+
+from utils.dates import (
+    day_end_iso,
+    day_start_iso,
+    duty_to_iso,
+    parse_iso_date,
+    validate_date_range,
+)
+from utils.user import build_full_name
+from utils.handover import parse_handover_time
+
+__all__ = [
+    "day_start_iso",
+    "day_end_iso",
+    "duty_to_iso",
+    "parse_iso_date",
+    "validate_date_range",
+    "build_full_name",
+    "parse_handover_time",
+]