feat(api): UDT-003 registro de usuarios — backend completo (Phases 1-6)

- Domain: Usuario.ForCreation factory, UsernameAlreadyExistsException, IUsuarioRepository extendido
- Application: CreateUsuarioCommand/Validator/Handler, UsuarioCreatedDto, AuthOptions password policy
- Infrastructure: UsuarioRepository.ExistsByUsernameAsync + AddAsync (INSERT OUTPUT INSERTED.Id), RoleClaimType="rol" en TokenValidationParameters
- Api: UsuariosController POST api/v1/users [Authorize(Roles="admin")], ExceptionFilter mapea UsernameAlreadyExistsException + SqlException 2627 → 409
- Tests (unit): 43 tests — 33 validator + 10 handler (107 total, green)
- Tests (integration): 7 tests CreateUsuarioEndpoint — 401/403/400/201/409/race/e2e (green)
- Fix: TestWebAppFactory.ConfigureTestServices reemplaza SqlConnectionFactory singleton con CS de test correcto

src/api/SIGCM2.Infrastructure/Persistence/UsuarioRepository.cs

@@ -56,6 +56,44 @@ public sealed class UsuarioRepository : IUsuarioRepository
         return MapRow(row);
     }
 
+    public async Task<bool> ExistsByUsernameAsync(string username, CancellationToken ct = default)
+    {
+        const string sql = """
+            SELECT COUNT(1) FROM dbo.Usuario WHERE Username = @Username
+            """;
+
+        await using var connection = _connectionFactory.CreateConnection();
+        await connection.OpenAsync(ct);
+
+        var count = await connection.ExecuteScalarAsync<int>(sql, new { Username = username });
+        return count > 0;
+    }
+
+    public async Task<int> AddAsync(Usuario usuario, CancellationToken ct = default)
+    {
+        // DF handles: Activo (1), PermisosJson ('[]'), FechaCreacion (GETDATE())
+        const string sql = """
+            INSERT INTO dbo.Usuario (Username, PasswordHash, Nombre, Apellido, Email, Rol)
+            OUTPUT INSERTED.Id
+            VALUES (@Username, @PasswordHash, @Nombre, @Apellido, @Email, @Rol)
+            """;
+
+        await using var connection = _connectionFactory.CreateConnection();
+        await connection.OpenAsync(ct);
+
+        var id = await connection.ExecuteScalarAsync<int>(sql, new
+        {
+            usuario.Username,
+            usuario.PasswordHash,
+            usuario.Nombre,
+            usuario.Apellido,
+            usuario.Email,
+            usuario.Rol
+        });
+
+        return id;
+    }
+
     private static Usuario MapRow(UsuarioRow row)
         => new(
             id: row.Id,