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

src/api/SIGCM2.Domain/Entities/Usuario.cs

@@ -0,0 +1,36 @@
+namespace SIGCM2.Domain.Entities;
+
+public sealed class Usuario
+{
+    public int Id { get; }
+    public string Username { get; }
+    public string PasswordHash { get; }
+    public string Nombre { get; }
+    public string Apellido { get; }
+    public string? Email { get; }
+    public string Rol { get; }
+    public string PermisosJson { get; }
+    public bool Activo { get; }
+
+    public Usuario(
+        int id,
+        string username,
+        string passwordHash,
+        string nombre,
+        string apellido,
+        string? email,
+        string rol,
+        string permisosJson,
+        bool activo)
+    {
+        Id = id;
+        Username = username;
+        PasswordHash = passwordHash;
+        Nombre = nombre;
+        Apellido = apellido;
+        Email = email;
+        Rol = rol;
+        PermisosJson = permisosJson;
+        Activo = activo;
+    }
+}

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") { }
+}

src/api/SIGCM2.Domain/SIGCM2.Domain.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <RootNamespace>SIGCM2.Domain</RootNamespace>
+  </PropertyGroup>
+
+</Project>