feat(iam): add @PrePersist callback to auto-populate User.createdAt

Add onCreate() method with @PrePersist annotation that sets createdAt
to LocalDateTime.now() on first persist, consistent with other entities
(UserTariff, CommunityTariff, etc.). Guard prevents overwriting existing
values. Two new tests verify auto-population and idempotency.

eeg_backend/src/main/java/at/mueller/eeg/backend/iam/domain/User.java

@@ -65,6 +65,13 @@ public class User implements UserDetails {
     @Column(nullable = false)
     private boolean enabled = false;
 
+    @PrePersist
+    protected void onCreate() {
+        if (createdAt == null) {
+            createdAt = LocalDateTime.now();
+        }
+    }
+
     @Override
     @NonNull
     public Collection<? extends GrantedAuthority> getAuthorities() {

eeg_backend/src/test/java/at/mueller/eeg/backend/iam/domain/UserTest.java

@@ -33,4 +33,28 @@ class UserTest {
 
         assertNull(user.getCreatedAt(), "createdAt should default to null (not auto-generated)");
     }
+
+    @Test
+    void prePersistSetsCreatedAt() {
+        User user = new User();
+
+        LocalDateTime before = LocalDateTime.now();
+        user.onCreate();
+        LocalDateTime after = LocalDateTime.now();
+
+        assertNotNull(user.getCreatedAt(), "createdAt must be set by @PrePersist callback");
+        assertFalse(user.getCreatedAt().isBefore(before), "createdAt must not be before test start");
+        assertFalse(user.getCreatedAt().isAfter(after), "createdAt must not be after test end");
+    }
+
+    @Test
+    void prePersistDoesNotOverwriteExistingCreatedAt() {
+        User user = new User();
+        LocalDateTime fixed = LocalDateTime.of(2025, 1, 1, 12, 0);
+        user.setCreatedAt(fixed);
+
+        user.onCreate();
+
+        assertEquals(fixed, user.getCreatedAt(), "createdAt must not be overwritten if already set");
+    }
 }