[bug] Boost stats are not updated when post gets un-boosted

[firescry]

Describe the bug with a clear and concise description of what the bug is.

Boost stats for post are not updated when post gets un-boosted by account from other server. It is especially visible when someone boosts a post multiple times - all that boosts accumulate.

Screenshot showing 4 boosts from the same account in Mastodon for Android app

What's your GoToSocial Version?

v0.17.3

GoToSocial Arch

arm64

What happened?

Account on external server boosted and un-boosted a post several times. Stats for that post were not updated on un-boost.

What you expected to happen?

When someone un-boosts a post, I would expect that stats for that post on my instance are updated accordingly, and their account is no longer shown among those who boosted the post.

How to reproduce it?

Two accounts are required:

• an account on GoToSocial server, let's call it a@gts
• an account on another server, let's call it b@external

Steps to reproduce:

1. a@gts follows b@external
2. b@external boosts a post
3. a@gts observes on their instance that the post was boosted by b@external
4. b@external un-boost the post
5. a@gts still sees that the post is boosted by b@external in the post's statistics

Anything else we need to know?

Stats are properly updated on my instance when I un-boost a post from account on my instance.

[tsmethurst]

Right, thanks! I think we still have only a stubbed implementation of handling incoming Undo Announce activities, which probably explains this behavior :P