feat(groups): prevent deletion of groups referenced by active invites

Adds GROUP_HAS_ACTIVE_INVITES error code and guards UserService.deleteGroup()
with a 409 conflict when any active (non-revoked, non-expired, non-exhausted)
invite token still holds the group UUID.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java

@@ -52,6 +52,8 @@ public enum ErrorCode {
     INVITE_REVOKED,
     /** The invite has passed its expiry date. 410 */
     INVITE_EXPIRED,
+    /** A group cannot be deleted because one or more active invites reference it. 409 */
+    GROUP_HAS_ACTIVE_INVITES,
 
     // --- Auth ---
     /** The request is not authenticated. 401 */

backend/src/main/java/org/raddatz/familienarchiv/user/InviteTokenRepository.java

@@ -24,4 +24,7 @@ public interface InviteTokenRepository extends JpaRepository<InviteToken, UUID>
 
     @Query("SELECT t FROM InviteToken t ORDER BY t.createdAt DESC")
     List<InviteToken> findAllOrderedByCreatedAt();
+
+    @Query("SELECT CASE WHEN COUNT(t) > 0 THEN true ELSE false END FROM InviteToken t JOIN t.groupIds g WHERE g = :groupId AND t.revoked = false AND (t.expiresAt IS NULL OR t.expiresAt > CURRENT_TIMESTAMP) AND (t.maxUses IS NULL OR t.useCount < t.maxUses)")
+    boolean existsActiveWithGroupId(@Param("groupId") UUID groupId);
 }

backend/src/main/java/org/raddatz/familienarchiv/user/UserService.java

@@ -37,6 +37,7 @@ public class UserService {
 
     private final AppUserRepository userRepository;
     private final UserGroupRepository groupRepository;
+    private final InviteTokenRepository inviteTokenRepository;
     private final PasswordEncoder passwordEncoder;
     private final AuditService auditService;
 
@@ -288,6 +289,10 @@ public class UserService {
 
     @Transactional
     public void deleteGroup(UUID id) {
+        if (inviteTokenRepository.existsActiveWithGroupId(id)) {
+            throw DomainException.conflict(ErrorCode.GROUP_HAS_ACTIVE_INVITES,
+                    "Cannot delete group " + id + " — referenced by one or more active invites");
+        }
         groupRepository.deleteById(id);
     }
 }

backend/src/test/java/org/raddatz/familienarchiv/user/UserServiceTest.java

@@ -36,6 +36,7 @@ class UserServiceTest {
 
     @Mock AppUserRepository userRepository;
     @Mock UserGroupRepository groupRepository;
+    @Mock InviteTokenRepository inviteTokenRepository;
     @Mock PasswordEncoder passwordEncoder;
     @Mock AuditService auditService;
     @InjectMocks UserService userService;
@@ -903,6 +904,29 @@ class UserServiceTest {
         assertThat(result.getPermissions()).containsExactlyInAnyOrder("READ_ALL", "WRITE_ALL");
     }
 
+    // ─── deleteGroup ──────────────────────────────────────────────────────────
+
+    @Test
+    void deleteGroup_throwsConflict_whenActiveInviteReferencesGroup() {
+        UUID groupId = UUID.randomUUID();
+        when(inviteTokenRepository.existsActiveWithGroupId(groupId)).thenReturn(true);
+
+        assertThatThrownBy(() -> userService.deleteGroup(groupId))
+                .isInstanceOf(DomainException.class)
+                .extracting(e -> ((DomainException) e).getCode())
+                .isEqualTo(ErrorCode.GROUP_HAS_ACTIVE_INVITES);
+    }
+
+    @Test
+    void deleteGroup_deletesGroup_whenNoActiveInviteReferencesGroup() {
+        UUID groupId = UUID.randomUUID();
+        when(inviteTokenRepository.existsActiveWithGroupId(groupId)).thenReturn(false);
+
+        userService.deleteGroup(groupId);
+
+        verify(groupRepository).deleteById(groupId);
+    }
+
     @Test
     void createGroup_withNullPermissions_savesGroupWithEmptyPermissionSet() {
         org.raddatz.familienarchiv.user.GroupDTO dto = new org.raddatz.familienarchiv.user.GroupDTO();