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/bdlconfig.json

@@ -173,6 +173,25 @@
     ],
     "minify": { "enabled": true }
   },
+  {
+    "context": "intranet:specialties",
+    "outputFileName": "wwwroot/web/fis.specialties.de.js",
+    "inputFiles": [
+      "js/intranet/modules/fis.specialties_txt_de.js",
+      "js/intranet/modules/fis.specialties.js"
+    ],
+    "minify": { "enabled": true }
+  },
+  {
+    "context": "intranet:specialties",
+    "outputFileName": "wwwroot/web/fis.specialties.min.css",
+    "inputFiles": [
+      "css/intranet/oci_variables.scss",
+      "css/intranet/fis_variables.scss",
+      "js/intranet/modules/fis.specialties.scss"
+    ],
+    "minify": { "enabled": true }
+  },
   {
     "context": "intranet:usr",
     "outputFileName": "wwwroot/web/fis.usr.de.js",