Add superuser as part of OpenKlantServiceManager

[swrichards]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 38.46154% with 8 lines in your changes missing coverage. Please review.

Project coverage is 94.84%. Comparing base (f1745d2) to head (6a38032).

Files with missing lines	Patch %	Lines
src/openklant2/tests/helpers.py	38.46%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1419      +/-   ##
===========================================
- Coverage    94.85%   94.84%   -0.02%     
===========================================
  Files         1049     1049              
  Lines        38837    38849      +12     
===========================================
+ Hits         36840    36845       +5     
- Misses        1997     2004       +7     

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