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/appsettings.json

@@ -42,11 +42,16 @@
     "fuchs_intranet_TOTP": "MANAGED_BY_KEYVAULT",
     "SMS_APIKey": "MANAGED_BY_KEYVAULT",
     "Mailer": {
-      "BaseUrl": "https://api.processweb.de",
+      "BaseUrl": "https://processweb.de",
       "AccountId": "7da3c1e6-4180-42d6-9a60-5a09d005cce6",
       "Token": "MANAGED_BY_KEYVAULT",
       "ServerId": "1192f3bf-57e5-4fd2-bb5d-3b7b983a2d98",
-      "Enabled": true
+      "Enabled": true,
+      "Attachments": {
+        "MaxFileBytes": 10485760,
+        "MaxMessageBytes": 20971520,
+        "MaxCount": 10
+      }
     },
     "Email": {
       "OverrideRecipient": ""