fix: OnlyOffice/Preview zeigt immer aktuelle Version (kein Cache)

Drei Cache-Ebenen gefixt:
- Vue Router: :key=fullPath erzwingt Komponenten-Neuaufbau bei
  jeder Navigation (kein Wiederverwenden alter Instanzen)
- Frontend: Cache-Bust Parameter an Preview + OnlyOffice API-Calls
- Backend: No-Cache Headers (Cache-Control, Pragma) auf Preview-Endpunkt

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/office.py

@@ -16,6 +16,13 @@ from app.models.settings import AppSettings
 @api_bp.route('/files/<int:file_id>/preview', methods=['GET'])
 @token_required
 def preview_file(file_id):
+    from flask import after_this_request
+    @after_this_request
+    def add_no_cache(response):
+        response.headers['Cache-Control'] = 'no-cache, no-store, must-revalidate'
+        response.headers['Pragma'] = 'no-cache'
+        return response
+
     user = request.current_user
     f, err = _get_file_or_403(file_id, user, 'read')
     if err: