[#296] Avoid breaking items relations to zaken

[SilviaAmAm]

Fixes #296
Fixes #309

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.71%. Comparing base (1136374) to head (9dd8dd1).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #300      +/-   ##
==========================================
+ Coverage   77.54%   77.71%   +0.17%     
==========================================
  Files         220      223       +3     
  Lines        6827     6951     +124     
  Branches      133      132       -1     
==========================================
+ Hits         5294     5402     +108     
- Misses       1409     1426      +17     
+ Partials      124      123       -1     

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[SilviaAmAm]

With the issue about changing how we cache zaken, this may no longer be needed (#309). I need to think about it.

[SilviaAmAm]

Decided: still needed for the resync process.