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/FuchsDatabase.sqlproj

@@ -127,6 +127,7 @@
     <Build Include="dbo\Tables\fds__mfr_invoicefiles.sql" />
     <Build Include="dbo\Tables\fds__log.sql" />
     <Build Include="dbo\Tables\fds__invoices.sql" />
+    <Build Include="dbo\Tables\fds__invoice_file_backups.sql" />
     <Build Include="dbo\Tables\fds__invoice_servicerequests.sql" />
     <Build Include="dbo\Tables\fds__invoice_items.sql" />
     <Build Include="dbo\Tables\fds__invoice_details.sql" />
@@ -357,6 +358,7 @@
     <Build Include="dbo\Stored Procedures\fds__admin_logdebug.sql" />
     <Build Include="dbo\Stored Procedures\fds__admin_logdebug_ext.sql" />
     <Build Include="dbo\Stored Procedures\fds__admin_getReportCatalog.sql" />
+    <Build Include="dbo\Stored Procedures\fds__admin_backupAndClearInvoiceFile.sql" />
     <Build Include="dbo\Stored Procedures\_BackupAndClearInvoiceFile.sql" />
     <Build Include="dbo\User Defined Types\json_data_maxU.sql" />
     <Build Include="dbo\User Defined Types\hash_256.sql" />

Fuchs_Database/dbo/Stored Procedures/fds__admin_backupAndClearInvoiceFile.sql

@@ -0,0 +1,62 @@
+-- =============================================
+-- Author:		Dr. Stefan Ott
+-- Create date: 2026-09-17
+-- Description:	App-facing counterpart to the manual [dbo].[_BackupAndClearInvoiceFile] maintenance
+--              tool (see that proc's header comment and Docs/Decisions/0025's "Alternatives
+--              considered"). That proc is explicitly manual-only and leaves no application log
+--              entry, so it is never called from here — this is a separate proc for the hidden
+--              admin/backupclearinvoicefile action (fds_sys > 4 gate, IntranetController.Admin.cs),
+--              which logs through the normal ILogger/DB-log-sink pipeline (ADR 0024) instead.
+--
+--              Copies the invoice's current [file] bytes into [dbo].[fds__invoice_file_backups],
+--              then clears [file] so the document regenerates on next read (ADR 0025). Both steps
+--              run in one transaction. Only a finalised, non-external invoice qualifies — the same
+--              precondition ADR 0025 puts on regeneration: clearing a draft's file would let the
+--              next read persist a draft render (fds__setInvoiceFile writes WHERE [file] IS NULL)
+--              that then blocks the real final PDF forever; an external (MFR) document cannot be
+--              regenerated at all. The calling service checks this too; this is defense in depth.
+--              An empty OUTPUT rowset means nothing was backed up/cleared (invoice missing, not
+--              finalised, external, or [file] already NULL) — rows-affected can't be used for this
+--              because of SET NOCOUNT ON (same reasoning as fds__setInvoiceFile, ADR 0025).
+-- =============================================
+CREATE PROCEDURE [dbo].[fds__admin_backupAndClearInvoiceFile]
+	@authuser varchar(25),
+	@Id       varchar(10)
+AS
+BEGIN
+	SET NOCOUNT ON;
+	-- The clear must never commit without its backup row: XACT_ABORT turns any statement-level
+	-- error inside the transaction into a doomed transaction, and the CATCH rolls back and re-raises
+	-- so the caller sees an error instead of an OUTPUT row (same pattern as the mfr__updt__* procs).
+	SET XACT_ABORT ON;
+
+	INSERT INTO [dbo].[fds__admin_activity] ([activity], [authuser], [info])
+		VALUES ('fds__admin_backupAndClearInvoiceFile', @authuser,
+			(SELECT * FROM (VALUES(@Id)) AS z([id]) FOR JSON PATH, INCLUDE_NULL_VALUES, WITHOUT_ARRAY_WRAPPER));
+
+	BEGIN TRY
+	BEGIN TRANSACTION;
+
+	INSERT INTO [dbo].[fds__invoice_file_backups] ([InvoiceId], [DocumentName], [file], [BackedUpBy])
+		SELECT [Id], [DocumentName], [file], @authuser
+		FROM [dbo].[fds__invoices]
+		WHERE [Id] = @Id AND [file] IS NOT NULL
+		  AND [DateFinalized] IS NOT NULL AND ISNULL([IsExternal], 0) = 0;
+
+	UPDATE [dbo].[fds__invoices]
+		SET [file] = NULL
+		OUTPUT inserted.[Id]
+		WHERE [Id] = @Id AND [file] IS NOT NULL
+		  AND [DateFinalized] IS NOT NULL AND ISNULL([IsExternal], 0) = 0;
+
+	COMMIT TRANSACTION;
+	END TRY
+	BEGIN CATCH
+		IF @@TRANCOUNT > 0 ROLLBACK TRANSACTION;
+
+		INSERT INTO dbo.[fds__admin_dberrors] ([UserName], [ErrorNumber], [ErrorState], [ErrorSeverity], [ErrorLine], [ErrorProcedure], [ErrorMessage], [ErrorDateTime])
+		VALUES (SUSER_SNAME(), ERROR_NUMBER(), ERROR_STATE(), ERROR_SEVERITY(), ERROR_LINE(), ERROR_PROCEDURE(), ERROR_MESSAGE(), GETDATE());
+
+		THROW;
+	END CATCH;
+END

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);