fix: loyalty_tier default 'bronze' in guestUpsert (constraint requires bronze/silver/gold/platinum)

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

backend/src/services/guestUpsert.ts

@@ -88,8 +88,8 @@ export async function upsertGuestFromBooking(params: {
     // No existing guest found — create new
     const { firstName, lastName, middleName } = parseName(guestName)
     await db.query(
-      `INSERT INTO guests (hotel_id, first_name, last_name, middle_name, email, phone)
-       VALUES ($1, $2, $3, $4, $5, $6)`,
+      `INSERT INTO guests (hotel_id, first_name, last_name, middle_name, email, phone, loyalty_tier)
+       VALUES ($1, $2, $3, $4, $5, $6, 'bronze')`,
       [
         hotelId,
         firstName,