Order messages by creation date, not indexing date

[AvivRubys]

Right now, we only save the indexing timestamp of each post, i.e. the timestamp that we received the firehose message in:

bluesky-hebrew-feed/src/subscription.ts

Lines 19 to 27 in ea0ba26

	return {
	uri: create.uri,
	author: create.author,
	cid: create.cid,
	replyTo: create.record.reply?.parent.uri,
	replyRoot: create.record.reply?.root.uri,
	indexedAt: new Date().toISOString(),
	language,
	};

That's maybe interesting to save, but why would we display posts by indexing date rather than by actual creation date? The CreateOp record also has a createdAt field, why don't we use it instead? This boils down to -

1. Add a new column for the new timestamp and populate it in the firehose subscription
2. Figure out a backfill strategy - take the existing indexedAt value? access the api and get the real value?
3. Change the feeds to order based on the new creation date column