Simplify user management

src/models/sheet.go

@@ -15,7 +15,6 @@ type Sheet struct {
 	FilePath    string         `json:"file_path" gorm:"not null"`
 	FileSize    int64          `json:"file_size"`
 	FileHash    uint64         `json:"file_hash"`
-	User        User           `json:"user" gorm:"foreignKey:UserID"`
 	CreatedAt   time.Time      `json:"created_at"`
 	UpdatedAt   time.Time      `json:"updated_at"`
 	DeletedAt   gorm.DeletedAt `json:"-" gorm:"index"`