Publication model: created_at, updated_at

[atomotic]

I suggest adding two datetime fields to the Publication model: created_at and updated_at. These fields should not be populated via API values but only on the application side.
They can be quite useful for filtering the publications table.

lcp-server/pkg/stor/publication.go

Lines 15 to 24 in a746270

	type Publication struct {
	gorm.Model
	UUID string `json:"uuid" validate:"required,uuid4_rfc4122" gorm:"uniqueIndex"`
	Title string `json:"title,omitempty"`
	EncryptionKey []byte `json:"encryption_key"`
	Location string `json:"location" validate:"required,url"`
	ContentType string `json:"content_type"`
	Size uint32 `json:"size"`
	Checksum string `json:"checksum" validate:"required,base64"`
	}