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/modules/fis.admin_txt_de.js

@@ -75,6 +75,8 @@ let $adt = {
     , tm_to: 'Empfänger'
     , tm_subject: 'Betreff'
     , tm_body: 'Nachricht'
+    , tm_attachpdf: 'PDF-Anhang mitsenden'
+    , tm_attachpdf_note: 'Hängt ein aus Betreff und Nachricht erzeugtes Test-PDF (Briefbogen-Layout) an und prüft so auch den Anhang-Pfad des Mailers.'
     , tm_hint: 'Der Versand nutzt die normale E-Mail-Pipeline; bei gesetztem OverrideRecipient wird die Nachricht dorthin umgeleitet.'
     , tm_sent: 'Test-E-Mail übergeben.'
     , tm_failed: 'Test-E-Mail konnte nicht gesendet werden.'