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/date_add.sql

@@ -2,6 +2,9 @@
 -- Author:		<Author,,Name>
 -- Create date: <Create Date, ,>
 -- Description:	<Description, ,>
+--              INLINE = OFF: scalar UDF inlining of this IF/nested-UDF body (-> AddBusinessDays)
+--              killed the dashboard widget queries with error 596 ("session is in the kill
+--              state", class 21). Called with constant arguments, so per-row cost is negligible.
 -- =============================================
 CREATE FUNCTION [dbo].[date_add]
 (
@@ -10,6 +13,7 @@ CREATE FUNCTION [dbo].[date_add]
 	,@shift_to_first bit = 1
 )
 RETURNS date
+WITH INLINE = OFF
 AS
 BEGIN
 	DECLARE @Ret date = @date;