fix(backend): add locking on relationship count constraints

[fatih-acar]

This covers relationship min_count, max_count constraints as well as cardinality one relationships.

Fixes #7972

[coderabbitai]

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[codspeed-hq]

CodSpeed Performance Report

Merging #7973 will not alter performance

_{Comparing fac-fix-issue-7972 (0ecaa2d) with stable (c4863c2)¹}

Summary

✅ 12 untouched

Footnotes

1. No successful run was found on stable (747cc3d) during the generation of this report, so c4863c2 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩