fix(invites): validate groupIds existence in createInvite — throw GROUP_NOT_FOUND

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

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

@@ -53,6 +53,9 @@ public class InviteService {
         Set<UUID> groupIds = new HashSet<>();
         if (dto.getGroupIds() != null && !dto.getGroupIds().isEmpty()) {
             List<UserGroup> groups = userService.findGroupsByIds(dto.getGroupIds());
+            if (groups.size() != dto.getGroupIds().size()) {
+                throw DomainException.notFound(ErrorCode.GROUP_NOT_FOUND, "One or more group IDs do not exist");
+            }
             groups.forEach(g -> groupIds.add(g.getId()));
         }
 

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

@@ -156,6 +156,20 @@ class InviteServiceTest {
         assertThat(result.getGroupIds()).contains(g.getId());
     }
 
+    @Test
+    void createInvite_throwsGroupNotFound_whenSubmittedGroupIdDoesNotExist() {
+        UUID unknownGroupId = UUID.randomUUID();
+        when(userService.findGroupsByIds(List.of(unknownGroupId))).thenReturn(List.of());
+
+        CreateInviteRequest req = new CreateInviteRequest();
+        req.setGroupIds(List.of(unknownGroupId));
+
+        assertThatThrownBy(() -> inviteService.createInvite(req, admin))
+                .isInstanceOf(DomainException.class)
+                .extracting(e -> ((DomainException) e).getCode())
+                .isEqualTo(ErrorCode.GROUP_NOT_FOUND);
+    }
+
     // ─── redeemInvite ─────────────────────────────────────────────────────────
 
     @Test