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/season/tab_settings.html.twig

@@ -1,6 +1,5 @@
 <div class="row">
     <div class="col-md-6 col-12">
-        <h4 class="mb-3">{{ 'Settings'|trans }}</h4>
         {{ form(form) }}
     </div>
 </div>