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_Database/dbo/Tables/fds__invoice_file_backups.sql

@@ -0,0 +1,13 @@
+CREATE TABLE [dbo].[fds__invoice_file_backups] (
+    [uid]          BIGINT          IDENTITY (1, 1) NOT NULL,
+    [InvoiceId]    VARCHAR (10)    NOT NULL,
+    [DocumentName] VARCHAR (50)    NULL,
+    [file]         VARBINARY (MAX) NOT NULL,
+    [BackedUpAt]   DATETIME        CONSTRAINT [DF_fds__invoice_file_backups_BackedUpAt] DEFAULT (getutcdate()) NOT NULL,
+    [BackedUpBy]   VARCHAR (25)    NULL,
+    CONSTRAINT [PK_fds__invoice_file_backups] PRIMARY KEY CLUSTERED ([uid] ASC)
+);
+
+GO
+CREATE NONCLUSTERED INDEX [fds__invoice_file_backups__ix1]
+    ON [dbo].[fds__invoice_file_backups]([InvoiceId] ASC);