dns/ddclient: validate full model on backend change

[fichtner]

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Is your feature request related to a problem? Please describe.

When switching backends settings for accounts might be unavailable which causes indeterministic errors with the other backend.

Describe the solution you'd like

As suggested do a full model validation on the backend change and complain about accounts not having the correct settings. It's only possible to delete these then to make the migration proceed, but it's good enough.

Describe alternatives you've considered

Leaving as is leads to users flip-flopping and complaining about having it misconfigured.

Additional context

N/A