fix: add ProductService.getById() to resolve pre-existing compile error

SupplierProductController references productService.getById() but
ProductService was missing this delegation method. Pre-existing bug
uncovered when running mvn compile for v1.1 verification.

cfc-backend/src/main/java/com/etotem/cfc/service/ProductService.java

@@ -36,6 +36,10 @@ public class ProductService {
     @Resource
     private AssessmentProductService assessmentProductService;
 
+    public Product getById(Long id) {
+        return productMapper.selectById(id);
+    }
+
     public Result<Map<String, Object>> list(ProductListQueryDTO query, Long userId) {
         Page<Product> page = new Page<>(query.getPage(), query.getSize());
         LambdaQueryWrapper<Product> wrapper = new LambdaQueryWrapper<Product>()