[Backend] Add relations ref to observable representations (#7419)

[CelineSebe]

Proposed changes

• register relationsRef

Related issues

• 7419

Checklist

• [ ] I consider the submitted work as finished
• [ ] I tested the code for its functionality
• [ ] I wrote test cases for the relevant uses case (coverage and e2e)
• [ ] I added/update the relevant documentation (either on github or on notion)
• [ ] Where necessary I refactored code to improve the overall quality

Further comments

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 67.36%. Comparing base (a8f9419) to head (a26aeb3).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #7474 +/- ## ========================================== + Coverage 67.34% 67.36% +0.02% ========================================== Files 563 563 Lines 69688 69722 +34 Branches 5890 5892 +2 ========================================== + Hits 46928 46970 +42 + Misses 22760 22752 -8 ```

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