feat: Implement admin-only specialties management interface

- Added MailerAttachmentPolicy for validating email attachments against server rules.
- Introduced fis.specialties.js for managing hidden maintenance actions via an admin interface.
- Created fis.specialties.scss for styling the specialties management UI.
- Added German localization for specialties management in fis.specialties_txt_de.js.
- Developed CSS for specialties management in fis.specialties.css.
- Implemented server-side stored procedure fds__admin_backupAndClearInvoiceFile for backing up and clearing invoice files.
- Created fds__invoice_file_backups table to store backup records of invoice files.

Fuchs/js/intranet/fis_main_go.js

@@ -2,5 +2,5 @@
     $fis.notifications.init();
     $fis.draft.init();
     $fis.ov();
-    $fis.addAdminMenuButtonsIfAuthorized();
+    $fis.addAdminMenuButtonsIfAuthorized().then($fis.openModuleFromPath);
 });