fix: configure CI database connection properly

- Add dotenv loading to Django settings
- Update CI workflow to use correct environment variables
- Set POSTGRES_* variables instead of DATABASE_URL
- Add environment variables to all Django management commands
- Fixes CI test failures due to database connection issues

app/stiftung/utils/date_utils.py

@@ -0,0 +1,34 @@
+from __future__ import annotations
+
+from datetime import date as _date
+from typing import Optional, Union
+
+try:
+    from django.utils.dateparse import parse_date as _parse_date
+except Exception:  # pragma: no cover - django not loaded in some tools
+    _parse_date = None  # type: ignore
+
+DateLike = Union[_date, str, None]
+
+
+def ensure_date(value: DateLike) -> Optional[_date]:
+    """Return a date from a date or ISO string; None stays None.
+
+    - Accepts datetime.date, 'YYYY-MM-DD' string, or None.
+    - Returns None if parsing fails or value falsy.
+    """
+    if not value:
+        return None
+    if isinstance(value, _date):
+        return value
+    if isinstance(value, str):
+        if _parse_date is None:
+            return None
+        return _parse_date(value)
+    return None
+
+
+def get_year_from_date(value: DateLike) -> Optional[int]:
+    """Extract year from date or ISO string, else None."""
+    d = ensure_date(value)
+    return d.year if d else None