feat: add unitMappings field to User and Product models

backend/prisma/schema.prisma

@@ -28,6 +28,7 @@ model User {
   pantryItems PantryItem[]
   mealPlanEntries MealPlanEntry[]
   receiptAliases  ReceiptAlias[]
+  unitMappings     UnitMapping[]
 }
 
 model Product {
@@ -54,6 +55,7 @@ model Product {
   categoryId        Int?
   categoryRef       Category?       @relation(fields: [categoryId], references: [id], onDelete: SetNull)
   isPrivate         Boolean         @default(false)
+  unitMappings      UnitMapping[]
 }
 
 model Category {