feat(backend): implement pricing engine for loyalty points and 3-level promotions

apps/backend/prisma/schema.prisma

@@ -82,6 +82,7 @@ model Customer {
   pictureUrl  String?
   email       String?
   phone       String?
+  points      Int      @default(0)
   orders      Order[]
   reviews     Review[]
   createdAt   DateTime @default(now())
@@ -116,12 +117,27 @@ model Coupon {
   updatedAt   DateTime  @updatedAt
 }
 
+model Promotion {
+  id          Int       @id @default(autoincrement())
+  name        String
+  level       Int
+  type        String
+  value       Float
+  minPurchase Float     @default(0)
+  isActive    Boolean   @default(true)
+  startDate   DateTime?
+  endDate     DateTime?
+  createdAt   DateTime  @default(now())
+  updatedAt   DateTime  @updatedAt
+}
+
 model StoreSettings {
   id                Int      @id @default(1)
   shippingFee       Float    @default(35)
   bankName          String   @default("KASIKORNBANK (KBANK)")
   bankAccountName   String   @default("HomAroma Co., Ltd.")
   bankAccountNumber String   @default("012-3-45678-9")
+  pointsEarnRate    Float    @default(1.0)
   updatedAt         DateTime @updatedAt
 }
 
@@ -131,10 +147,22 @@ model Order {
   customerEmail   String
   customerPhone   String
   shippingAddress String
-  totalAmount     Float
-  shippingFee     Float       @default(35)
+  
+  subtotal        Float       @default(0)
+  promoLevel1Name String?
+  promoLevel1Disc Float       @default(0)
+  promoLevel2Name String?
+  promoLevel2Disc Float       @default(0)
+  
   couponCode      String?
   discountAmount  Float       @default(0)
+  
+  pointsUsed      Int         @default(0)
+  pointsDiscount  Float       @default(0)
+  pointsEarned    Int         @default(0)
+  
+  shippingFee     Float       @default(35)
+  totalAmount     Float
   status          String      @default("PENDING_PAYMENT")
   paymentSlipUrl  String?