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_full.sql

@@ -0,0 +1,37 @@
+
+-- =============================================
+-- Author:		<Author,,Name>
+-- Create date: <Create Date, ,>
+-- Description:	<Description, ,>
+-- =============================================
+CREATE FUNCTION [dbo].[fds__fn_InvoicePaymentAmount_full]
+(
+	@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)
+	), ci as(
+		SELECT [InvId] = d.[StornoTo_InvId], [amount] = ISNULL(i.[InvoiceBalance],0.0) * -1 FROM [dbo].[fds__invoices] as i JOIN [dbo].[fds__invoice_details] as d on i.[Id] = d.[InvId] where d.[StornoTo_InvId] = @InvID  AND i.[IsFinal] = 1
+	), tp as (
+		SELECT [InvId], [amount] from bs where [invid] = @InvID
+		union
+		SELECT [InvId], [amount] from ci
+	), b as (
+		SELECT 
+			[InvID]	= @InvID
+			,[amount]	= SUM(ISNULL(tp.[amount], 0.0))
+		FROM tp
+		WHERE tp.[invid] = @InvID
+	)
+	SELECT TOP(1) @RES = [amount] FROM b;
+
+	RETURN ISNULL(@RES, 0.0);
+
+END