feat(udt-001): domain layer with Usuario entity

src/api/SIGCM2.Domain/Exceptions/InvalidCredentialsException.cs

@@ -0,0 +1,11 @@
+namespace SIGCM2.Domain.Exceptions;
+
+/// <summary>
+/// Thrown when login credentials are invalid (user not found, wrong password, or inactive).
+/// Deliberately vague to prevent user enumeration.
+/// </summary>
+public sealed class InvalidCredentialsException : Exception
+{
+    public InvalidCredentialsException()
+        : base("Credenciales inválidas") { }
+}