Add in_reply_to Actor to Activity audience

[pfefferle]

Part of #1698 fixes #1712

Proposed changes:

• Add the Actor of the replied post in cc or to, depending on the visibility.

Other information:

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

Testing instructions:

• Publish a new post with a reply-block
• Load the post-url with ?activitypub
• Check the JSON output for the Actor of the replied URL

Changelog entry

• Automatically create a changelog entry from the details below.

Changelog Entry Details

Significance

• Patch
• Minor
• Major

Type

• Added - for new features
• Changed - for changes in existing functionality
• Deprecated - for soon-to-be removed features
• Removed - for now removed features
• Fixed - for any bug fixes
• Security - in case of vulnerabilities

Message

The actor of the replied-to post is now included in cc or to based on the post's visibility.

[Copilot]

Pull Request Overview

This PR adds functionality to include the actor referenced by "in_reply_to" in the activity's audience.

• Integrates retrieval of the "in_reply_to" actor via a new HTTP call.
• Updates audience composition to add the "in_reply_to" actor across different content visibility cases.