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_candidates.html.twig

@@ -1,10 +1,8 @@
 <div class="row">
     <div class="col-md-6 col-12">
-        <div class="d-flex align-items-center mb-3">
-            <h4 class="mb-0 pe-2">{{ 'Candidates'|trans }}</h4>
-            <a class="link"
-               href="{{ path('tvdt_backoffice_add_candidates', {seasonCode: season.seasonCode}) }}">{{ 'Add Candidate'|trans }}
-            </a>
+        <div class="mb-3">
+            <a class="btn btn-sm btn-outline-primary"
+               href="{{ path('tvdt_backoffice_add_candidates', {seasonCode: season.seasonCode}) }}">{{ 'Add Candidate'|trans }}</a>
         </div>
         <ul class="mb-3">
             {% for candidate in season.candidates %}