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_InvoicePaymentAmount.sql

@@ -0,0 +1,30 @@
+-- =============================================
+-- Author:		<Author,,Name>
+-- Create date: <Create Date, ,>
+-- Description:	<Description, ,>
+-- =============================================
+CREATE FUNCTION [dbo].[fds__fn_InvoicePaymentAmount]
+(
+	@InvID varchar(15)
+)
+RETURNS numeric(9,2)
+AS
+BEGIN
+	DECLARE @RES numeric(9,2);
+	DECLARE @invoice_idlist [dbo].[fds__tt__idlist_vchar];
+	INSERT INTO @invoice_idlist VALUES(@invid);
+
+	With bs as (
+		SELECT * FROM [dbo].[fds__fn_bankingtransactions](null,null, null, null,@invoice_idlist)
+	), b as (
+		SELECT 
+			[InvID]	= @InvID
+			,[amount]	= SUM(ISNULL(bs.[amount], 0.0))
+		FROM bs
+		WHERE bs.[invid] = @InvID
+	)
+	SELECT TOP(1) @RES = [amount] FROM b;
+
+	RETURN ISNULL(@RES, 0.0);
+
+END