refactor(products): remove brand field from Product model and update related DTO and service

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

backend/src/products/dto/update-product.dto.ts

@@ -22,11 +22,6 @@ export class UpdateProductDto {
   @MaxLength(191)
   subcategory?: string;
 
-  @IsOptional()
-  @IsString()
-  @MaxLength(191)
-  brand?: string;
-
   @IsOptional()
   @IsNumber()
   categoryId?: number | null;

backend/src/products/products.service.ts

@@ -164,7 +164,6 @@ export class ProductsService {
       normalizedName?: string;
       canonicalName?: string;
       category?: string | null;
-      brand?: string | null;
       categoryId?: number | null;
     } = {};
 
@@ -193,10 +192,6 @@ export class ProductsService {
       updateData.category = data.category.trim() || null;
     }
 
-    if (typeof data.brand === 'string') {
-      updateData.brand = data.brand.trim() || null;
-    }
-
     if ('categoryId' in data) {
       updateData.categoryId = data.categoryId ?? null;
     }