🐛 [#626] Use raw_id_fields for klantcontact and other missing FK r…

[OlhaZahoruiko]

…elations

Fixes #626

Changes

• Fix the Betrokkenen bij klantcontact admin timeout by rendering the klantcontact relation with raw_id_fields instead of loading all Klantcontact records into a dropdown
• Optimize other FK relations in the admin with raw_id_fields
• Add a Django system check to detect unoptimized FK relations in registered admins and inlines

Checklist

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

• Architectural design record

  • If a design decision was made that changes functionality, create an architectural design record for it here

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.43503% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.00%. Comparing base (4cd0082) to head (51a0d5c).

Files with missing lines	Patch %	Lines
src/openklant/utils/checks.py	98.48%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #661      +/-   ##
==========================================
+ Coverage   97.98%   98.00%   +0.02%     
==========================================
  Files         238      241       +3     
  Lines       14069    14245     +176     
==========================================
+ Hits        13785    13961     +176     
  Misses        284      284              

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