Fix offset-naive/aware datetime error in nag checker

Make last_nag_at timezone-aware before subtracting from the tz-aware
current_time, and store future nag timestamps with timezone.utc to
prevent the mismatch recurring.

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

core/adaptive_meds.py

@@ -10,7 +10,7 @@ This module handles:
 
 import json
 import uuid
-from datetime import datetime, timedelta, time
+from datetime import datetime, timedelta, time, timezone
 from typing import Optional, Dict, List, Tuple
 import core.postgres as postgres
 from core.tz import user_now, user_today_for
@@ -291,6 +291,8 @@ def should_send_nag(
     nag_interval = settings.get("nag_interval_minutes", 15)
 
     if last_nag:
+        if isinstance(last_nag, datetime) and last_nag.tzinfo is None:
+            last_nag = last_nag.replace(tzinfo=timezone.utc)
         time_since_last_nag = (current_time - last_nag).total_seconds() / 60
         if time_since_last_nag < nag_interval:
             return False, f"Too soon ({int(time_since_last_nag)} < {nag_interval} min)"
@@ -335,7 +337,7 @@ def record_nag_sent(user_uuid: str, med_id: str, scheduled_time):
 
         postgres.update(
             "medication_schedules",
-            {"nag_count": new_nag_count, "last_nag_at": datetime.utcnow()},
+            {"nag_count": new_nag_count, "last_nag_at": datetime.now(timezone.utc)},
             {"id": schedule["id"]},
         )