Backfill database with historical data

[Ovyerus]

Use https://github.com/bluesky-social/proposals/tree/main/0006-sync-iteration to backfill the database with records that were created before the service was running/while it was offline.

To consider:

1. When should the service decide to search the network for records it doesn't have?
2. How to handle that when coming back from some downtime. (Probably not actually necessary due to persisting the seq from the firehose so we can resume nicely.)
3. How to optimise the fetching so we're not being a dick, and ideally be smart about it so we're not asking for stuff we already have (not sure if the proposal/existing lexicons cover that, I need to do some reading).