Store delivery and archive times without the monotonic reading (closes #529)

finished_at, archived_at and the updated_at stamp a re-dispatch
writes came from a bare time.Now(). The SQLite driver stores a time
as time.Time.String(), which appends the monotonic clock reading
(" m=+…") to a time read from time.Now(). All three now come from
GORM's own clock, NowFunc(), which stamps created_at, so they are
stored in the same form. The tests for each write read the stored
text back.

Model: opus-5-5

internal/delivery/target_database_archive.go

@@ -277,7 +277,11 @@ func (w *archiveWriter) write(
 		}
 	}
 
-	row.ArchivedAt = time.Now()
+	// GORM's clock, the one that stamps created_at in the other
+	// databases, so this is stored in the same form. A bare time.Now()
+	// carries a monotonic clock reading, which the driver would store
+	// as an " m=+…" suffix.
+	row.ArchivedAt = w.db.NowFunc()
 
 	err := w.db.Create(&row).Error
 	if err != nil {