feat(iam): add UserRepository.findAllByOrderByCreatedAtDesc()

Add method to retrieve all users ordered by creation date descending.
This supports the admin user management overview feature.

Tests:
- UserRepositoryTest: verify method exists and returns List<User>

eeg_backend/src/main/java/at/mueller/eeg/backend/iam/repository/UserRepository.java

@@ -19,4 +19,6 @@ public interface UserRepository extends JpaRepository<User, UUID> {
     Optional<User> findByVerificationToken(String token);
 
     Optional<User> findByPasswordResetToken(String token);
+
+    List<User> findAllByOrderByCreatedAtDesc();
 }

eeg_backend/src/test/java/at/mueller/eeg/backend/iam/repository/UserRepositoryTest.java

@@ -0,0 +1,37 @@
+package at.mueller.eeg.backend.iam.repository;
+
+import at.mueller.eeg.backend.iam.domain.User;
+import org.junit.jupiter.api.Test;
+
+import java.lang.reflect.Method;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class UserRepositoryTest {
+
+    @Test
+    void findAllByOrderByCreatedAtDesc_methodExists() throws NoSuchMethodException {
+        Method method = UserRepository.class.getMethod("findAllByOrderByCreatedAtDesc");
+        assertNotNull(method);
+        assertEquals(List.class, method.getReturnType());
+    }
+
+    @Test
+    void findAllByOrderByCreatedAtDesc_returnsUserList() throws NoSuchMethodException {
+        Method method = UserRepository.class.getMethod("findAllByOrderByCreatedAtDesc");
+        assertNotNull(method, "findAllByOrderByCreatedAtDesc method not found on UserRepository");
+
+        Class<?> returnType = method.getReturnType();
+        assertEquals(List.class, returnType, "Method should return List");
+
+        java.lang.reflect.Type genericReturnType = method.getGenericReturnType();
+        assertTrue(genericReturnType instanceof java.lang.reflect.ParameterizedType,
+                "Generic return type should be ParameterizedType");
+
+        java.lang.reflect.ParameterizedType parameterizedType =
+                (java.lang.reflect.ParameterizedType) genericReturnType;
+        Class<?> actualTypeArgument = (Class<?>) parameterizedType.getActualTypeArguments()[0];
+        assertEquals(User.class, actualTypeArgument, "Method should return List<User>");
+    }
+}