Removed nullable types and `init` properties, replacing them with standard getters and setters in various entity classes. Updated properties like `DocumentPath`, `SendingProfile`, and `SignatureHost` to be non-nullable and added required attributes.
Modified the project file to include `net462` as a target framework and added a reference for `System.ComponentModel.Annotations`. Removed the `IUnique<int>` interface from several classes to simplify uniqueness management.
These changes improve data integrity and usability across the entity classes.
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