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/AddBusinessDays.sql

@@ -0,0 +1,9 @@
+CREATE FUNCTION[dbo].[AddBusinessDays](@Date DATE,@n INT)
+RETURNS DATE AS 
+BEGIN
+DECLARE @d INT,@f INT,@DW INT;
+SET @f=CAST(abs(1^SIGN(DATEPART(DW, @Date)-(7-@@DATEFIRST))) AS BIT)
+SET @DW=DATEPART(DW,@Date)-(7-@@DATEFIRST)*(@f^1)+@@DATEFIRST*(@f&1)
+SET @d=4-SIGN(@n)*(4-@DW);
+RETURN DATEADD(D,@n+((ABS(@n)+(@d%(8+SIGN(@n)))-2)/5)*2*SIGN(@n)-@d/7,@Date);
+END