fix(task): keep DoneAt once written instead of clearing it

Done tasks never leave the done state (doStart, pause and markTaskError
all guard against Done), so the else branch clearing DoneAt was
unreachable dead code. DoneAt is now written once on completion and
kept for the task lifetime.

pkg/download/model.go

@@ -32,8 +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 is the time when the task completed; nil if it has never
+	// finished. A done task never leaves the done state, so the value is
+	// written once and kept.
 	DoneAt *time.Time `json:"doneAt"`
 
 	producer         production.Source
@@ -137,11 +138,11 @@ func (t *Task) MarshalJSON() ([]byte, error) {
 func (t *Task) updateStatus(status base.Status) {
 	t.UpdatedAt = time.Now()
 	t.Status = status
+	// A done task never leaves the done state, so there is no branch to clear
+	// DoneAt on other transitions.
 	if status == base.DownloadStatusDone {
 		doneAt := time.Now()
 		t.DoneAt = &doneAt
-	} else {
-		t.DoneAt = nil
 	}
 }