feat(task): record and display task completion time

Add DoneAt to the download task model: set when a task finishes and
cleared when it is downloaded again. The value flows through the API
model and task record, and is shown as "Completed at" in the task
drawer info tab next to the creation time.

pkg/download/model.go

@@ -32,6 +32,9 @@ type Task struct {
 	Progress  *Progress            `json:"progress"`
 	CreatedAt time.Time            `json:"createdAt"`
 	UpdatedAt time.Time            `json:"updatedAt"`
+	// DoneAt is the time when the task last completed; nil while the task has
+	// never finished or is being downloaded again.
+	DoneAt *time.Time `json:"doneAt"`
 
 	producer         production.Source
 	producerReceived int64
@@ -134,6 +137,12 @@ func (t *Task) MarshalJSON() ([]byte, error) {
 func (t *Task) updateStatus(status base.Status) {
 	t.UpdatedAt = time.Now()
 	t.Status = status
+	if status == base.DownloadStatusDone {
+		doneAt := time.Now()
+		t.DoneAt = &doneAt
+	} else {
+		t.DoneAt = nil
+	}
 }
 
 func (t *Task) clone() *Task {