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.

CLAUDE.md

@@ -38,7 +38,7 @@
 
 ## Services & Dependency Injection
 - Business logic lives in **DI-registered services** under `Fuchs/Services/` behind interfaces, injected into `IntranetController`. Do not reintroduce static God-classes or pass the controller into helpers.
-- Services: `IComService` (ProcessWeb Mailer API; honors the `Fuchs:Email:OverrideRecipient` dev safety net — see Configuration), `IPdfService`, `IInvoiceService`, `IReminderService`, `IReportService`, `IWidgetService`, `IBankingService`, `IMfrClientFactory`, `ISystemStatusService` (Admin module diagnostics: config snapshot + DB/Key Vault/blob/MFR connectivity probes + test-email; restricted to `fds_sys` > 4). Stateless ones are singletons; DB/request-scoped ones are scoped (see `Program.cs`).
+- Services: `IComService` (ProcessWeb Mailer API — contract at **https://processweb.de/api/mailer/fc**, read it and `Fuchs/Docs/Concepts/processweb-mailer-interface.md` before changing it. `ProcessWebComService` is a thin adapter over the OCORE_web `IPWMailerClientService` (`OCORE_web/comm/pwmailer/`, owns the HTTP contract; ADR 0029) with `FuchsMailerClient` overriding `ValidateMessage` for attachment limits; a send only counts as successful when the receipt's `attachmentsProcessed` matches the number sent (ADR 0028); honors the `Fuchs:Email:OverrideRecipient` dev safety net — see Configuration), `IPdfService`, `IInvoiceService`, `IReminderService`, `IReportService`, `IWidgetService`, `IBankingService`, `IMfrClientFactory`, `ISystemStatusService` (Admin module diagnostics: config snapshot + DB/Key Vault/blob/MFR connectivity probes + test-email; restricted to `fds_sys` > 4). Stateless ones are singletons; DB/request-scoped ones are scoped (see `Program.cs`).
 - `FdsInvoiceData` / `FdsReminderData` are **pure data holders**; load/persist/render belongs in services. No `Task.Run(...).Wait()` sync-over-async.
 - Data access is SQL-first via OCORE helpers + stored procedures (no EF Core).