fix(deploy): quote FlexibleTimeWindow.Mode as string (EarlyValidation)

YAML 1.1 implicit conversion: bare 'OFF' parses as boolean false.
AWS::Scheduler::Schedule.FlexibleTimeWindow.Mode requires the string
'OFF' or 'FLEXIBLE'. CFN's EarlyValidation hook rejects the boolean,
causing 'Failed to create changeset' on every deploy attempt.

Verified by manual create-change-set against the SAM-packaged
template: Mode: false -> FAILED; Mode: "OFF" -> CREATE_COMPLETE
with 4 changes (BotFunction modify, LolscheduleDailyPushSchedule add,
SchedulerExecutionRole policy modify, MonthlyBudget remove).

template.yaml

@@ -221,7 +221,7 @@ Resources:
       Description: Fires lolschedule daily-push handler at 01:00 UTC (08:00 ICT)
       ScheduleExpression: "cron(0 1 * * ? *)"
       ScheduleExpressionTimezone: UTC
-      FlexibleTimeWindow: { Mode: OFF }
+      FlexibleTimeWindow: { Mode: "OFF" }  # quoted: bare OFF → YAML 1.1 boolean false → EarlyValidation rejects
       State: ENABLED
       Target:
         Arn: !GetAtt BotFunction.Arn