Refactor: unify role constants under new Role class

Replaced all usages of ReceiverRole with the new Role class in EnvelopeGenerator.Domain.Constants. Removed ReceiverRole.cs and added Role.cs with PreAuth and FullyAuth constants. Updated all [Authorize] attributes and role checks in controllers and authentication logic to use Role.FullyAuth and Role.PreAuth. This centralizes role management for improved maintainability and clarity.

EnvelopeGenerator.Web/Controllers/ReadOnlyController.cs

@@ -34,7 +34,7 @@ namespace EnvelopeGenerator.Web.Controllers
         }
 
         [HttpPost]
-        [Authorize(Roles = ReceiverRole.FullyAuth)]
+        [Authorize(Roles = Role.FullyAuth)]
         [Obsolete("Use MediatR")]
         public async Task<IActionResult> CreateAsync([FromBody] EnvelopeReceiverReadOnlyCreateDto createDto)
         {