Transaction enrichment should handle the initial backfill as a special case

[zachgoll]

Currently, transaction enrichment via Synth will loop through all user transaction entries and attempt to auto-assign categories, clean up the name on the transaction, and assign a merchant.

Since many users have thousands of transactions that need to be backfilled, we should give the user more control over which transactions will be enriched (and consume Synth credits). Currently, new self hosted instances will have data enrichment turned off by default, but no controls around what data will be synced when this is turned on: