fix(invite): reject invalidated invites in acceptInvite

Same invalidatedAt gap as getInviteInfo: a superseded invite (status
still 'pending', invalidatedAt set) could still be used to create an
account and join the household.

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

backend/src/main/java/com/recipeapp/household/HouseholdService.java

@@ -205,7 +205,9 @@ public class HouseholdService {
         HouseholdInvite invite = householdInviteRepository.findByInviteCode(code)
                 .orElseThrow(() -> new ResourceNotFoundException("Invite not found or invalid"));
 
-        if ("used".equals(invite.getStatus()) || invite.getExpiresAt().isBefore(Instant.now())) {
+        if ("used".equals(invite.getStatus())
+                || invite.getInvalidatedAt() != null
+                || invite.getExpiresAt().isBefore(Instant.now())) {
             throw new ResourceNotFoundException("Invite not found or invalid");
         }