refactor: GetOverview 5 次独立查询合并为单次 GROUP BY

- 新增 PostRepo.GetOverviewByUserID 单次 CASE WHEN + GROUP BY 查询
- Service 层 GetOverview 从 5 次 DB 查询减少为 1 次
- 接口 postStore 新增 GetOverviewByUserID 声明

internal/service/repository.go

@@ -37,6 +37,7 @@ type postStore interface {
 	FindPageable(keyword string, offset, limit int) ([]model.Post, int64, error)
 	FindAdminPageable(keyword, status string, offset, limit int) ([]model.Post, int64, error)
 	FindByUserIDAndStatus(userID uint, status string, offset, limit int) ([]model.Post, int64, error)
+	GetOverviewByUserID(userID uint) (totalPosts, draftCount, pendingCount, approvedCount, rejectedCount int64, err error)
 	FindPendingRevisions(keyword string, offset, limit int) ([]model.Post, int64, error)
 	Update(post *model.Post) error
 	SoftDelete(id uint) error