fix: 删除用户后无法重新注册同名用户 (#317)

* fix: 删除用户后无法重新注册同名用户

用户删除改为硬删除，解决软删除导致 username 唯一约束冲突的问题。
用户表无业务数据关联，无需保留已删除记录。
同时添加迁移清理历史软删除用户数据。

* fix: Delete 方法在未匹配到行时返回 ErrNotFound

* fix: v5 迁移 Down 函数返回不可逆错误，避免误导回滚成功

* fix: UserRepository.Delete 使用 Unscoped 执行硬删除

软删除仅设置 deleted_at 而不移除行，导致 username 唯一约束冲突，
删除用户后无法重新注册同名用户。添加 Unscoped() 确保物理删除。

Author: solarhell

internal/repository/sqlite/migrations.go

@@ -166,6 +166,23 @@ var migrations = []Migration{
 			return nil
 		},
 	},
+	{
+		Version:     5,
+		Description: "Hard-delete soft-deleted users to free username unique constraint",
+		Up: func(db *gorm.DB) error {
+			result := db.Exec("DELETE FROM users WHERE deleted_at != 0")
+			if result.Error != nil {
+				return result.Error
+			}
+			if result.RowsAffected > 0 {
+				log.Printf("[Migration] Purged %d soft-deleted users", result.RowsAffected)
+			}
+			return nil
+		},
+		Down: func(db *gorm.DB) error {
+			return errors.New("migration v5 is irreversible: hard-deleted users cannot be restored")
+		},
+	},
 }
 
 func isMySQLDuplicateIndexError(err error) bool {

internal/repository/sqlite/user.go

@@ -35,14 +35,16 @@ func (r *UserRepository) Update(u *domain.User) error {
 	return r.db.gorm.Save(model).Error
 }
 
+// Delete 硬删除用户。用户表无业务数据关联，软删除会导致 username 唯一约束冲突，无法重新注册同名用户。
 func (r *UserRepository) Delete(tenantID uint64, id uint64) error {
-	now := time.Now().UnixMilli()
-	return r.db.gorm.Model(&User{}).
-		Where("id = ? AND tenant_id = ?", id, tenantID).
-		Updates(map[string]any{
-			"deleted_at": now,
-			"updated_at": now,
-		}).Error
+	result := r.db.gorm.Unscoped().Where("id = ? AND tenant_id = ?", id, tenantID).Delete(&User{})
+	if result.Error != nil {
+		return result.Error
+	}
+	if result.RowsAffected == 0 {
+		return domain.ErrNotFound
+	}
+	return nil
 }
 
 func (r *UserRepository) GetByID(tenantID uint64, id uint64) (*domain.User, error) {