feat: address PR review comments — unassign/sync bank questions, blank quiz creation, deactivate redirect, remove duplicate tab titles

- Use Symfony ObjectMapper for BankQuestion/BankAnswer → Question/Answer copy (#[Map(if: false)] on id, season, etc.)
- Track created Question on BankQuestionUsage (nullable FK, onDelete: SET NULL) for unassign/sync support
- Add unassign route: removes the Question copy + usage record
- Add sync route: pushes bank question edits to a finalized-not-started quiz copy
- Auto-sync non-finalized quiz copies on bank question edit; flash warning for finalized-not-started
- Add blank quiz creation (no XLSX required) with new route + template
- Deactivate quiz button now stays on the quiz overview page (redirect_quiz hidden field)
- Remove duplicate h4 titles below the tab bar on all season tabs
- Add migration for bank_question_usage.question_id
- Add Dutch translations for all new strings

templates/backoffice/quiz/tab_overview.html.twig

@@ -38,6 +38,7 @@
         {% if quiz is same as (season.activeQuiz) %}
             <form action="{{ path('tvdt_backoffice_enable', {seasonCode: season.seasonCode, quiz: 'null'}) }}" method="POST">
                 <input type="hidden" name="_token" value="{{ csrf_token('enable_quiz') }}">
+                <input type="hidden" name="redirect_quiz" value="{{ quiz.id }}">
                 <button type="submit" class="btn btn-secondary rounded-0 rounded-start">
                     {{ 'Deactivate Quiz'|trans }}
                 </button>