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_isInvoiceCancelled.sql

@@ -0,0 +1,28 @@
+-- =============================================
+-- Author:		<Author,,Name>
+-- Create date: <Create Date, ,>
+-- Description:	<Description, ,>
+-- =============================================
+CREATE FUNCTION [dbo].[fds__fn_isInvoiceCancelled]
+(
+	@InvId varchar(20)
+)
+RETURNS bit
+AS
+BEGIN
+	DECLARE @res bit;
+	
+	--SET @res = CASE WHEN EXISTS (SELECT 0 FROM [dbo].[fds__invoices] as i2 JOIN [dbo].[fds__invoice_details] as id2 ON i2.[id] = id2.[InvId] where id2.[StornoTo_InvId] = @invId AND i2.[IsFinal] = 1 ) THEN 1 ELSE 0 END;
+
+	with storno as(
+		SELECT [amount] = SUM(ISNULL(ORIG.[invoicebalance],0.0)), [stornoamount] = SUM(ISNULL(i2.[invoicebalance],0.0) * -1.0)
+			FROM [dbo].[fds__invoices] as ORIG
+				LEFT JOIN [dbo].[fds__invoice_details] as id2 ON id2.StornoTo_InvId = ORIG.[id] and id2.StornoTo_InvId = @invId
+				LEFT JOIN [dbo].[fds__invoices] as i2 on i2.[id] = id2.[InvId] AND i2.[IsFinal] = 1
+			WHERE ORIG.[Id] = @invId
+	)
+	SELECT @res = CASE WHEN [amount] = [stornoamount] THEN 1 ELSE 0 END from storno;
+
+	RETURN cast(ISNULL(@res,0) as bit);
+
+END