fix: Make statusModel `created_at` fields be ISO 8601 strings

This makes the 'Reactions to this post' status (seen when viewing
a status context) send the correct data type for `created_at` fields.

https://docs.joinmastodon.org/entities/Account/#created_at
https://docs.joinmastodon.org/entities/Status/#created_at

packages/backend/src/server/api/mastodon/endpoints/status.ts

@@ -388,7 +388,7 @@ export function statusModel(
 	emojis: MastodonEntity.Emoji[],
 	content: string,
 ) {
-	const now = Math.floor(new Date().getTime() / 1000);
+	const now = new Date().toISOString();
 	return {
 		id: "9atm5frjhb",
 		uri: "https://http.cat/404", // ""