Follow: Set local user as actor in Accept activities

[obenland]

Follow up to #1759.

I noticed Accept activities getting rejected by Ghost, with the message The signer and the actor do not match.. This PR restores setting the actor for the Accept activity to the local user that's being followed.

Proposed changes:

• Reverts changes to actor property in Rework Follower Structure #1759.
• Updates unit test to cover that case.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Apply this PR to a test site that's accessible from the internet.
• From another instance, send a new Follow request to your test site.
• Make sure the Follow is successful.

[Copilot]

Pull Request Overview

This PR reverts the earlier change to the actor property in the Accept activity and enhances the unit test to verify that the local user is correctly set as the activity’s actor.

• Restores original behavior of setting the actor field to the local user ID in queue_accept
• Updates test_queue_accept to use the Actors and Followers collections, stub remote metadata, and assert on the local actor
• Adds cleanup for the remote metadata filter

[pfefferle]

We should have a patch release for that one!!!

[pfefferle]

🫣