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_id.sql

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+CREATE   FUNCTION [dbo].[fds__fn_invoice_id] (
+)
+RETURNS varchar(10)
+AS
+BEGIN
+  DECLARE @NewAccount varchar(5) = [dbo].[ocms_fn_generatePassword] (8,0,1,0);
+
+  WHILE EXISTS( SELECT * FROM [dbo].[fds__invoices] WITH (SERIALIZABLE) WHERE [id] = @NewAccount)
+	BEGIN
+		SET @NewAccount = [dbo].[ocms_fn_generatePassword] (8,0,1,0);
+	END
+
+  RETURN @NewAccount;
+END;