DataPurgeController was deleting Jobs without first clearing the nullable
Appointments.JobId FK, causing FK_Appointments_Jobs_JobId violations.
Fix nulls out the FK on any linked appointments before the DELETE runs.
Also applies migration AddAllowCustomFormulas (AllowCustomFormulas column
on SubscriptionPlanConfigs for custom formula pricing feature gating).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
