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/dates.py

@@ -0,0 +1,46 @@
+"""Date and ISO helpers for duty ranges and API validation."""
+
+import re
+from datetime import date, datetime, timezone
+
+
+def day_start_iso(d: date) -> str:
+    """ISO 8601 start of calendar day UTC: YYYY-MM-DDT00:00:00Z."""
+    return d.isoformat() + "T00:00:00Z"
+
+
+def day_end_iso(d: date) -> str:
+    """ISO 8601 end of calendar day UTC: YYYY-MM-DDT23:59:59Z."""
+    return d.isoformat() + "T23:59:59Z"
+
+
+def duty_to_iso(d: date, hour_utc: int, minute_utc: int) -> str:
+    """ISO 8601 with Z for start of duty on date d at given UTC time."""
+    dt = datetime(d.year, d.month, d.day, hour_utc, minute_utc, 0, tzinfo=timezone.utc)
+    return dt.strftime("%Y-%m-%dT%H:%M:%SZ")
+
+
+# ISO date YYYY-MM-DD
+_ISO_DATE_RE = re.compile(r"^\d{4}-\d{2}-\d{2}$")
+
+
+def parse_iso_date(s: str) -> date | None:
+    """Parse YYYY-MM-DD string to date. Returns None if invalid."""
+    if not s or not _ISO_DATE_RE.match(s.strip()):
+        return None
+    try:
+        return date.fromisoformat(s.strip())
+    except ValueError:
+        return None
+
+
+def validate_date_range(from_date: str, to_date: str) -> None:
+    """Validate from_date and to_date are YYYY-MM-DD and from_date <= to_date.
+
+    Raises:
+        ValueError: With a user-facing message if invalid.
+    """
+    if not _ISO_DATE_RE.match(from_date or "") or not _ISO_DATE_RE.match(to_date or ""):
+        raise ValueError("Параметры from и to должны быть в формате YYYY-MM-DD")
+    if from_date > to_date:
+        raise ValueError("Дата from не должна быть позже to")