[MARXAN-446] planning area assigning

[Dyostiq]

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marxan – ./app

🔍 Inspect: https://vercel.com/vizzuality1/marxan/7MvkZKLtffrQp53bQVahYH8zEqn6
✅ Preview: https://marxan-git-feature-marxan-446-planning-area-748bbd-vizzuality1.vercel.app

marxan-storybook – ./app

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[hotzevzl]

in practice we could even delete this planning area straight away rather than orphaning it and letting it be GC'ed later on (but we'd need to do this in a transaction to make sure the delete can be rolled back if the successive update fails