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()));
         }