Add Fuchs_Database SSDT project (schema source of truth)

Adds the SQL Server Data Tools project for the fuchs_fds database — tables,
table types, functions and stored procedures that the backend calls (e.g.
fds__getInvoice, fds__merge_bankingtransactions, fds__tt__bankingtransactions,
fds__admin_getReportCatalog, fis_* auth). Build/model caches (bin, obj,
*.dbmdl, *.jfm, *.user) are git-ignored.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Fuchs_Database/dbo/Functions/fis_admin_checkUserExists.sql

@@ -0,0 +1,20 @@
+
+-- =============================================
+-- Author:		<Author,,Name>
+-- Create date: <Create Date, ,>
+-- Description:	<Description, ,>
+-- =============================================
+CREATE FUNCTION [dbo].[fis_admin_checkUserExists]
+(
+	@useraccount_id varchar(5)
+)
+RETURNS bit
+AS
+BEGIN
+	DECLARE @RET bit;
+
+	SET @RET = IIF(EXISTS (SELECT * FROM [dbo].[fis_useraccounts] as ua WHERE [useraccount_id] = @useraccount_id AND ua.[DateDeleted] is null AND ua.[DateDisabled] is null), 1, 0);
+
+	RETURN @RET;
+
+END