[feature] Serve bot accounts over AP as Service instead of Person

[tsmethurst]

Description

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This pull request updates our account serialization logic to take account of the fact that bot accounts should be served as actor type service, not actor type person, so that remotes can properly mark them as bot.

closes #3518

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
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