Function deriveResponseStatus

Derive the current response status for a record given today's date.

Rules (in priority order):

1. If response_skrivelse_id is set:
   • If gov_response_status === 'PARTIAL' (or open_recommendations > 0) → PARTIAL
   • Otherwise → RESPONDED
2. If deadline has elapsed and no response → OVERDUE
3. Otherwise → PENDING

Status is derived primarily from the presence of response_skrivelse_id: a record cannot be PENDING/OVERDUE if a skrivelse reference is recorded.