fix: handle datetime.date object for birth_year in ZIP export

PostgreSQL returns dob as datetime.date object, not string.
Changed from prof['dob'][:4] to prof['dob'].year

Error was: TypeError: 'datetime.date' object is not subscriptable

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/main.py

@@ -1599,7 +1599,7 @@ Datumsformat: YYYY-MM-DD
                     "email": prof.get('email'),
                     "sex": prof.get('sex'),
                     "height": float(prof['height']) if prof.get('height') else None,
-                    "birth_year": int(prof['dob'][:4]) if prof.get('dob') else None,
+                    "birth_year": prof['dob'].year if prof.get('dob') else None,
                     "goal_weight": float(prof['goal_weight']) if prof.get('goal_weight') else None,
                     "goal_bf_pct": float(prof['goal_bf_pct']) if prof.get('goal_bf_pct') else None,
                     "avatar_color": prof.get('avatar_color'),