keywords and intervention_areas can be orphans

The current implementation doesn't enforce keywords or intervention_areas to be linked to an Osbl or else when creating a 'osbl_creation'

I could change that, or add a cron job deleting orphans daily

The issue names no files, tests, or entry points. Start by locating the osbl_creation implementation and the handling of keywords and intervention_areas; clarify whether creation must enforce links or a scheduled cleanup is intended, then add coverage for the chosen behavior.

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