feat: add inventoryId to SalesInvoice and related models; implement createOrder functionality in POS service

prisma/schema.prisma

@@ -173,6 +173,7 @@ model Inventory {
   stockAdjustments       StockAdjustment[]   @relation("Inventory_Stock_Adjustments")
   stockMovements         StockMovement[]     @relation("StockMovement_Inventory")
   stockBalances          StockBalance[]      @relation("StockBalance_inventory")
+  salesInvoices          SalesInvoice[]      @relation("Inventory_SalesInvoices")
 
   @@map("Inventories")
 }
@@ -256,8 +257,8 @@ model PurchaseReceiptItem {
   createdAt   DateTime        @default(now()) @db.Timestamp(0)
   receiptId   Int
   productId   Int
-  product     Product         @relation("Product_PurchaseReceiptItems", fields: [productId], references: [id])
   receipt     PurchaseReceipt @relation("PurchaseReceipt_Items", fields: [receiptId], references: [id])
+  product     Product         @relation("Product_PurchaseReceiptItems", fields: [productId], references: [id])
 
   @@index([productId], map: "Purchase_Receipt_Items_productId_fkey")
   @@index([receiptId], map: "Purchase_Receipt_Items_receiptId_fkey")
@@ -272,9 +273,12 @@ model SalesInvoice {
   createdAt   DateTime           @default(now()) @db.Timestamp(0)
   updatedAt   DateTime           @updatedAt @db.Timestamp(0)
   customerId  Int?
+  inventoryId Int
   items       SalesInvoiceItem[] @relation("SalesInvoice_Items")
   customer    Customer?          @relation("Customer_Sales_Invoices", fields: [customerId], references: [id])
+  inventory   Inventory          @relation("Inventory_SalesInvoices", fields: [inventoryId], references: [id])
 
+  @@index([inventoryId], map: "Sales_Invoices_inventoryId_fkey")
   @@index([customerId], map: "Sales_Invoices_customerId_fkey")
   @@map("Sales_Invoices")
 }