Allow admins to merge topics [backend]

What does this MR do and why?

This MR is the first step of #366884 (closed).

• Implement the backend (controller, service) to allow admins to merge topics

The next steps of the implementation plan will be done in follow-up MRs.

🛠 with ❤ at Siemens

/cc @bufferoverflow

Screenshots or screen recordings

see #366884 (closed)

How to set up and validate locally

1. Controller: see spec/controllers/admin/topics_controller_spec.rb
2. Service: see spec/services/topics/merge_service_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.