feat(showcase): 去掉 Bento 灵感类型，只保留 hero / work

后端校验、分组、后台列表，以及后台页面与首页取图都只认
hero 和 work；启动时清掉遗留的 bento 记录。

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

backend/internal/repo/showcase_repo.go

@@ -62,9 +62,8 @@ func (r *ShowcaseRepository) Grouped(ctx context.Context) (map[string][]model.Sh
 	}
 
 	grouped := map[string][]model.ShowcaseItem{
-		"hero":  {},
-		"bento": {},
-		"work":  {},
+		"hero": {},
+		"work": {},
 	}
 	for _, item := range items {
 		grouped[item.Kind] = append(grouped[item.Kind], item)