- Updated `ReceiverGetOrCreateCommand` to include a required `IEnumerable<Signature>`.
- Modified `DocumentCreateCommand` to remove `DataAsByte` and enforce a required `DataAsBase64` property with immutability.
- Cleaned up SQL command formatting in `EnvelopeReceiverAddReadSQL`.
- Simplified document validation logic in `EnvelopeReceiverController` by removing redundant checks.
- Changed hardcoded connection string to a variable `_cnnStr` for improved security and flexibility.
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