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.js

@@ -187,7 +187,7 @@ let $adm = {
         ];
         if (e.overrideActive === true) { rows.push([$adt.override, e.overrideRecipient]); }
         // The pill only turns green once the credentials are verified against the Mailer API
-        // (probe 'email', GET /mailer/fc?fn=verify) — mailerEnabled alone is not proof they work.
+        // (probe 'email', GET /api/mailer/verify) — mailerEnabled alone is not proof they work.
         let card = $adm.probeCard(grid, $adt.mail, 'email', rows, probe);
         let ft = $$.dc('adm_card_ft', card);
         $$.bbtn($adt.testmail_btn, 'adm_btn adm_btn_primary').appendTo(ft).click($adm.testMailDlg);
@@ -221,11 +221,12 @@ let $adm = {
         $ocms.dlgform([
             { name: 'to', label: $adt.tm_to, type: 'email', required: true, value: ($ocms.auth.email || '') },
             { name: 'subject', label: $adt.tm_subject, type: 'string', required: true, value: $adt.tm_default_subject },
-            { name: 'body', label: $adt.tm_body, type: 'text', required: true, value: $adt.tm_default_body }
+            { name: 'body', label: $adt.tm_body, type: 'text', required: true, value: $adt.tm_default_body },
+            { name: 'attachpdf', label: $adt.tm_attachpdf, type: 'bool', required: true, value: true, note: $adt.tm_attachpdf_note }
         ], {
             title: $adt.testmail,
             button: $adt.testmail_btn,
-            size: [420, 560],
+            size: [420, 640],
             addcontent: $$.dc('adm_note').text($adt.tm_hint),
             url: $ocms.url('admin/testmail'),
             typedvalues: true,