Changed `EnvelopeReceiverReadOnlyDto` and `EnvelopeReceiverReadOnlyUpdateDto` from records to classes, converting properties to standard get/set accessors and adding documentation comments. Introduced a default value for `ChangedWhen` in `EnvelopeReceiverReadOnlyUpdateDto`.
Modified `ReceiverCreateDto` to a class structure, updating properties to use init-only accessors and encapsulating SHA256 hash logic in a private field. Retained the `AddedWhen` property with its default value.
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