Move account counters to separate table

[Gargron]

statuses_count is the most updated column probably among all tables, because it goes up every time you post a new status. By extracting it to a table with only a few columns, updates can run faster and the dead tuples left behind until next vacuum are smaller.

Fix #7908

Instructions:

1. Run SKIP_POST_DEPLOYMENT_MIGRATIONS=true rails db:migrate first
2. Restart Mastodon
3. Run rails db:migrate

[nightpool]

i think this should maybe be attributes[key]? or something similar, i can't remember the precise syntax offhand.

[Gargron]

That's if the counter was on the same table as before. We want to call the explicitly defined followers_count method here.

[Gargron]

Note: This query has been wrong for a long time. I think it hasn't been updated since ActivityPub was introduced. It would send PuSH subscriptions to ActivityPub accounts. It also would not correctly filter accounts that had local followers. So this is a net improvement.

[nightpool]

should be delegate :statuses_count, :statuses_count=, :following_count, :following_count=, :followers_count, :followers_count=, to: :account_stat

and then handle the case of nil account_stat in the set_account_stat hook

[nightpool]

%w(statuses following followers).flat_map { |i| [:"#{i}_count", :"#{i}_count=" } is even a little cuter, your call.

[nightpool]

per our conversation, this should handle migrating the counts from the existing record if there's no account_stat record.

[nightpool]

i think these should live on AccountStat and be delegated also

[nightpool]

and then we don't need to care about this method