fix(monitoring): handle timezone-naive datetimes in get_server_states_view

Fixes datetime comparison error when last_seen_at from database is
offset-naive (no timezone info) while 'now' is offset-aware (UTC).

This resolves the TypeError: can't subtract offset-naive and
offset-aware datetimes issue in integration tests.

app/services/monitoring.py

@@ -288,6 +288,9 @@ def get_server_states_view(db: Session, offline_after_minutes: int = 7):
     for s in servers:
         st = db.query(ServerState).filter(ServerState.server_id == s.id).first()
         last_seen = st.last_seen_at if st else None
+        # Handle timezone-naive datetimes from database
+        if last_seen and last_seen.tzinfo is None:
+            last_seen = last_seen.replace(tzinfo=timezone.utc)
         online = bool(last_seen and (now - last_seen).total_seconds() <= offline_after_minutes * 60)
         out.append({
             'server_id': s.id,