fix(invite): saveAndFlush invalidation before INSERT + set invalidated_at on accept

- createInvite: use saveAndFlush when invalidating existing invite so the
  UPDATE is guaranteed to hit the DB before the new INSERT, preventing
  duplicate key violation on uq_household_invite_active
- acceptInvite: also set invalidated_at when marking invite as used, so
  accepted invites are fully removed from the partial unique index and
  cannot block future invite creation

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

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

@@ -165,7 +165,7 @@ public class HouseholdService {
         householdInviteRepository.findByHouseholdIdAndInvalidatedAtIsNull(household.getId())
                 .ifPresent(existing -> {
                     existing.setInvalidatedAt(Instant.now());
-                    householdInviteRepository.save(existing);
+                    householdInviteRepository.saveAndFlush(existing);
                 });
 
         String code = generateInviteCode();
@@ -211,6 +211,7 @@ public class HouseholdService {
                 new UserAccount(email, name, passwordEncoder.encode(rawPassword)));
 
         invite.setStatus("used");
+        invite.setInvalidatedAt(Instant.now());
         householdInviteRepository.save(invite);
 
         Household household = invite.getHousehold();