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/wwwroot/web/fis.admin.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.'
@@ -272,7 +274,7 @@ 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);
@@ -306,11 +308,12 @@ 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,