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)

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;

Fuchs_Database/dbo/Functions/date_addcustom.sql

@@ -2,6 +2,8 @@
 -- Author:		<Author,,Name>
 -- Create date: <Create Date, ,>
 -- Description:	<Description, ,>
+--              INLINE = OFF: scalar UDF inlining (nested -> AddBusinessDays) killed the dashboard
+--              widget queries with error 596 ("session is in the kill state", class 21).
 -- =============================================
 CREATE FUNCTION [dbo].[date_addcustom]
 (
@@ -9,6 +11,7 @@ CREATE FUNCTION [dbo].[date_addcustom]
 	,@phrase varchar(10)
 )
 RETURNS date
+WITH INLINE = OFF
 AS
 BEGIN
 	DECLARE @returndate date, @numpart integer = TRY_PARSE(left(@phrase, patindex('%[^0-9]%', @phrase + '.') - 1) as integer), @defpart varchar(10) = LOWER(RIGHT(@phrase, LEN(@phrase) - patindex('%[^0-9]%', @phrase + '.') + 1));