feat: Implement API to GDPR delete users

[Janis4411]

This PR introduces a possible way to GDPR delete users via a API.

If we delete a user on openHPI we also have to delete the user on codeocean. Previously this was handled via a rake task that has to be triggered manually.

Part of SODEV-2997

[sentry]

Bug: Sequential calls to head :unauthorized in verify_token! and verify_signature! without a return can cause a DoubleRenderError if both checks fail.
_{Severity: MEDIUM}

Suggested Fix

Add an explicit return statement after each head :unauthorized call in both the verify_token! and verify_signature! methods to ensure execution is halted after the response is rendered.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: app/controllers/api/internal/users/deletions_controller.rb#L21-L22

Potential issue: In the `deletions_controller`, the `authenticate_request!` method
sequentially calls `verify_token!` and `verify_signature!`. If a token is invalid,
`verify_token!` calls `head :unauthorized` but does not halt execution. Control then
proceeds to `verify_signature!`. If the signature is also invalid, it too calls `head
:unauthorized`. This second attempt to render a response in the same request cycle
triggers an `ActionController::DoubleRenderError`. This will occur for any API request
that has both an invalid token and an invalid signature.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[codecov]

Codecov Report

❌ Patch coverage is 53.12500% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.25%. Comparing base (7876111) to head (3238780).

Files with missing lines	Patch %	Lines
...rollers/api/internal/users/deletions_controller.rb	50.00%	11 Missing ⚠️
app/jobs/user_cleanup_job.rb	60.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3224      +/-   ##
==========================================
- Coverage   70.33%   70.25%   -0.09%     
==========================================
  Files         214      216       +2     
  Lines        6839     6871      +32     
==========================================
+ Hits         4810     4827      +17     
- Misses       2029     2044      +15     

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