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/date_weekend.sql

@@ -0,0 +1,17 @@
+-- =============================================
+-- Author:		Dr. Stefan Ott
+-- Create date: 31.01.2013
+-- Description:	first day of week
+-- =============================================
+CREATE FUNCTION [dbo].[date_weekend]
+(
+	@TargetDate date
+)
+RETURNS date
+AS
+BEGIN
+	
+	DECLARE @Corretor smallint = -((@@DATEFIRST % 7) - 1);
+	DECLARE @ret date = CASE WHEN @TargetDate is null THEN null ELSE dateadd(d ,-((datepart(weekday,@TargetDate) - 1 - @Corretor)%7), @TargetDate) END;
+	return DATEADD(DAY, 6, @ret);
+END