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/Dockerfile

@@ -7,6 +7,7 @@ COPY src src
 RUN ./mvnw package -DskipTests -B
 
 FROM eclipse-temurin:21-jre-alpine
+RUN apk add --no-cache libwebp
 WORKDIR /app
 COPY --from=build /app/target/*.jar app.jar
 EXPOSE 8080

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();

backend/src/test/java/com/recipeapp/household/HouseholdServiceTest.java

@@ -507,11 +507,13 @@ class HouseholdServiceTest {
 
         when(householdMemberRepository.findByUserEmailIgnoreCase("sarah@example.com")).thenReturn(Optional.of(member));
         when(householdInviteRepository.findByHouseholdIdAndInvalidatedAtIsNull(any())).thenReturn(Optional.of(existingInvite));
+        when(householdInviteRepository.saveAndFlush(any(HouseholdInvite.class))).thenAnswer(i -> i.getArgument(0));
         when(householdInviteRepository.save(any(HouseholdInvite.class))).thenAnswer(i -> i.getArgument(0));
 
         householdService.createInvite("sarah@example.com");
 
         assertThat(existingInvite.getInvalidatedAt()).isNotNull();
-        verify(householdInviteRepository, times(2)).save(any(HouseholdInvite.class));
+        verify(householdInviteRepository).saveAndFlush(existingInvite);
+        verify(householdInviteRepository).save(any(HouseholdInvite.class));
     }
 }