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/Docs/ARCHITECTURE.md

@@ -196,8 +196,8 @@ Cookie-based authentication (`CookieAuthenticationDefaults`) with custom claims
 
 > **Status: DONE.** The services below are implemented and DI-registered in
 > `Program.cs`. The original extraction rationale is retained for reference /
-> history. `FuchsFdsEmail` → `IComService` (ProcessWeb Mailer API, inline
-> base64 attachments), `FuchsWidgets` → `IWidgetService`, `FuchsPdf` →
+> history. `FuchsFdsEmail` → `IComService` (ProcessWeb Mailer API, multipart
+> attachments — see `Docs/Concepts/processweb-mailer-interface.md`), `FuchsWidgets` → `IWidgetService`, `FuchsPdf` →
 > `IPdfService`, `Banking` → `IBankingService` (now MT940 **and** CAMT),
 > `FdsInvoiceData`/`FdsReminderData` → `IInvoiceService`/`IReminderService`
 > (data classes are now pure POCOs), `FuchsReports` → `IReportService`