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