feat(meal-plan): add servings field to MealPlanEntry and update related functionality

feat(products): implement bulk update for product categories

feat(recipes): add servings input to WriteRecipePage and update MealPlanClient for servings management

refactor(types): enhance Product and Category types with additional properties

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

@@ -0,0 +1,12 @@
+import { IsArray, IsInt, IsNumber, IsOptional, ArrayMinSize } from 'class-validator';
+
+export class BulkUpdateProductsDto {
+  @IsArray()
+  @ArrayMinSize(1)
+  @IsInt({ each: true })
+  ids: number[];
+
+  @IsOptional()
+  @IsNumber()
+  categoryId?: number | null;
+}