feat: add technical user deletion logic and adjust exception handling for encryption

[Phil91]

Description

• add delete technical user endpoint
• add technical user deletion process
• adjust exceptionhandling for encryption

Why

To provide the delete logic for the portal backend and handle missing encryption keys without killing the process worker

Issue

N/A

Checklist

• I have performed a self-review of my own code
• I have successfully tested my changes locally
• I have added tests that prove my changes work
• I have checked that new and existing tests pass locally with my changes
• I have commented my code, particularly in hard-to-understand areas
• I have added copyright and license headers, footers (for .md files) or files (for images)