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/fds__fn_invoice_customerid.sql

@@ -0,0 +1,19 @@
+-- =============================================
+-- Author:		<Author,,Name>
+-- Create date: <Create Date, ,>
+-- Description:	<Description, ,>
+-- =============================================
+CREATE FUNCTION [dbo].[fds__fn_invoice_customerid]
+(
+	@InvId varchar(15)
+)
+RETURNS bigint
+AS
+BEGIN
+	DECLARE @ret bigint;
+
+	SELECT TOP(1) @ret = [customerid] FROM dbo.[fds__invoices] WHERE [id] = @InvId;
+	
+	RETURN @ret;
+
+END