Give the digest window a way back

Deleting a message that Slack no longer has counts as done, so the window
rewinds instead of staying stuck. A reset button clears the mark by hand,
and the empty-run notice now says since the last dispatch instead of seven
days.

src/lib/admin-settings.ts

@@ -187,3 +187,18 @@ export async function performDeleteMessage(viewer: Viewer, formData: FormData):
 
 	return { status: 'ok', message: 'messageDeleted' }
 }
+
+export async function performResetWindow(viewer: Viewer): Promise<AdminFormState> {
+	await clearSetting('slack.lastSentAt')
+
+	await recordAudit({
+		actorId: viewer.id,
+		actorLabel: viewer.email,
+		action: 'settings.reportReset',
+		entity: 'setting',
+		entityId: 'slack',
+		data: {},
+	})
+
+	return { status: 'ok', message: 'windowReset' }
+}