feat: add location field to PantryItem model and update related functionality

Co-authored-by: Copilot <copilot@github.com>

backend/src/pantry/dto/create-pantry-item.dto.ts

@@ -1,7 +1,12 @@
-import { IsInt, IsPositive } from 'class-validator';
+import { IsInt, IsOptional, IsPositive, IsString, MaxLength } from 'class-validator';
 
 export class CreatePantryItemDto {
   @IsInt()
   @IsPositive()
   productId: number;
+
+  @IsOptional()
+  @IsString()
+  @MaxLength(50)
+  location?: string;
 }

backend/src/pantry/pantry.service.ts

@@ -33,7 +33,11 @@ export class PantryService {
     }
 
     return this.prisma.pantryItem.create({
-      data: { userId, productId: data.productId },
+      data: {
+        userId,
+        productId: data.productId,
+        location: data.location?.trim() || null,
+      },
       include: { product: true },
     });
   }