feat(helpdesk): Übergabe assignment workflow

- New statuses: under_review
- New DB fields: assigned_to, assigned_by, assigned_at, due_date (migration 012)
- HandoverNotificationService: emails on assign and review-request
- HandoverService: assign(), submitForReview(), resendNotification()
- Assign page: manager selects employee + due date, resend notification
- Create wizard: manager can assign during creation (step 1)
- Edit page: "Submit for review" button for assignee, assign link for manager
- List page: open/closed tabs, non-managers see only their assigned handovers
- Email templates: handover_assigned + handover_review_requested (de/en)
- i18n keys added for de and en

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/helpdesk/module.php

@@ -53,6 +53,7 @@ return [
         ['path' => 'helpdesk/handovers/create', 'target' => 'helpdesk/handovers/create'],
         ['path' => 'helpdesk/handovers/edit/{id}', 'target' => 'helpdesk/handovers/edit'],
         ['path' => 'helpdesk/handovers/bulk/{action}', 'target' => 'helpdesk/handovers/bulk'],
+        ['path' => 'helpdesk/handovers/assign/{id}', 'target' => 'helpdesk/handovers/assign'],
         ['path' => 'helpdesk/debitor-lookup-data', 'target' => 'helpdesk/debitor-lookup-data'],
         ['path' => 'helpdesk/handover-domains-data', 'target' => 'helpdesk/handover-domains-data'],
         ['path' => 'helpdesk/updates', 'target' => 'helpdesk/updates'],