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_monthend.sql

@@ -0,0 +1,18 @@
+
+
+-- =============================================
+-- Author:		<Author,,Name>
+-- Create date: <Create Date, ,>
+-- Description:	<Description, ,>
+-- =============================================
+CREATE FUNCTION [dbo].[date_monthend]
+(
+	@date date
+)
+RETURNS date
+AS
+BEGIN
+	DECLARE @monthend date = CASE WHEN @date is null THEN NULL ELSE DATEADD(DAY, -1, DATEADD(MONTH, 1, DATEFROMPARTS(YEAR(@date), MONTH(@date),1))) END;
+
+	return @monthend;
+END