Add AuthScopedValidator and register FluentValidation

Introduced AuthScopedValidator to enforce non-empty AddedWho on IAuthScoped entities with a clear error message. Registered all validators from the assembly in DI to enable automatic validation for authentication-scoped entities.

src/ReC.Application/Common/Validations/AuthScopedValidator.cs

@@ -0,0 +1,14 @@
+using FluentValidation;
+using ReC.Application.Common.Interfaces;
+
+namespace ReC.Application.Common.Validations;
+
+public class AuthScopedValidator : AbstractValidator<IAuthScoped>
+{
+    public AuthScopedValidator()
+    {
+        RuleFor(x => x.AddedWho)
+            .NotEmpty()
+            .WithMessage("The 'AddedWho' field is required. A missing value may indicate an API configuration issue. Please contact your system administrator for assistance.");
+    }
+}