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/handover.py

@@ -0,0 +1,37 @@
+"""Handover time parsing for duty schedule import."""
+
+import re
+from datetime import datetime, timezone
+
+# HH:MM or HH:MM:SS, optional space + timezone (IANA or "UTC")
+HANDOVER_TIME_RE = re.compile(
+    r"^\s*(\d{1,2}):(\d{2})(?::(\d{2}))?\s*(?:\s+(\S+))?\s*$", re.IGNORECASE
+)
+
+
+def parse_handover_time(text: str) -> tuple[int, int] | None:
+    """Parse handover time string to (hour_utc, minute_utc). Returns None on failure."""
+    m = HANDOVER_TIME_RE.match(text)
+    if not m:
+        return None
+    hour = int(m.group(1))
+    minute = int(m.group(2))
+    # second = m.group(3) ignored
+    tz_str = (m.group(4) or "").strip()
+    if not tz_str or tz_str.upper() == "UTC":
+        return (hour % 24, minute)
+    try:
+        from zoneinfo import ZoneInfo
+    except ImportError:
+        try:
+            from backports.zoneinfo import ZoneInfo  # type: ignore
+        except ImportError:
+            return None
+    try:
+        tz = ZoneInfo(tz_str)
+    except Exception:
+        return None
+    # Build datetime in that tz and convert to UTC
+    dt = datetime(2000, 1, 1, hour, minute, 0, tzinfo=tz)
+    utc = dt.astimezone(timezone.utc)
+    return (utc.hour, utc.minute)