autopush: Enforce must_use on Db's update_user

[data-sync-user]

This now returns a boolean of whether the operation actually occurred due to a conditional on the db update. As a safety precaution calling code should be forced to #[must_use] the result to encourage them appropriately handling when an update doesn’t occur.

This would have to be implemented with something like a wrapper struct (e.g. MustUse<T>) around the bool.

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