feat(pos): implement integrated full-screen POS point-of-sale system with inventory deduction and thermal receipt

apps/backend/prisma/schema.prisma

@@ -313,11 +313,12 @@ model NewsletterSubscriber {
 }
 
 model Order {
-  id              Int    @id @default(autoincrement())
-  customerName    String
-  customerEmail   String
-  customerPhone   String
-  shippingAddress String
+  id              Int     @id @default(autoincrement())
+  channel         String  @default("ONLINE") // "ONLINE" | "POS"
+  customerName    String?
+  customerEmail   String?
+  customerPhone   String?
+  shippingAddress String?
 
   subtotal        Float   @default(0)
   promoLevel1Name String?
@@ -332,9 +333,13 @@ model Order {
   pointsDiscount Float @default(0)
   pointsEarned   Int   @default(0)
 
-  shippingFee    Float   @default(35)
+  shippingFee    Float   @default(0)
   totalAmount    Float
   status         String  @default("PENDING_PAYMENT")
+  paymentMethod  String? @default("CASH")
+  receivedAmount Float?
+  changeAmount   Float?
+  cashierId      Int?
   paymentSlipUrl String?
   trackingNo     String?
 
@@ -344,6 +349,9 @@ model Order {
   items     OrderItem[]
   createdAt DateTime    @default(now())
   updatedAt DateTime    @updatedAt
+
+  @@index([channel])
+  @@index([createdAt])
 }
 
 model OrderItem {