fix(drafts): serialize draft session flushes and register reminders via fds__setReminder

Make invoice/reminder live-draft edits concurrency-safe with an explicit
DraftFlushResult, add the fds__setReminder stored procedure, and harden the
date helper functions. Includes concurrency and SQL contract tests.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Fuchs_Database/dbo/Functions/AddBusinessDays.sql

@@ -1,5 +1,9 @@
-CREATE FUNCTION[dbo].[AddBusinessDays](@Date DATE,@n INT)
-RETURNS DATE AS 
+-- INLINE = OFF: scalar UDF inlining of this @@DATEFIRST arithmetic (reached via date_add /
+-- date_addcustom) killed dashboard widget queries with error 596 ("session is in the kill state").
+CREATE FUNCTION[dbo].[AddBusinessDays](@Date DATE,@n INT)
+RETURNS DATE
+WITH INLINE = OFF
+AS
 BEGIN
 DECLARE @d INT,@f INT,@DW INT;
 SET @f=CAST(abs(1^SIGN(DATEPART(DW, @Date)-(7-@@DATEFIRST))) AS BIT)