CHW-5403 BillingPlanUpdateFailed

unable to update the plan assignment; retry shortly and contact support if the failure persists

Default message: unable to update the plan assignment; retry shortly and contact support if the failure persists
HTTP status: 500 (server)

Problem. /api/billing/checkout (Free path) returned 500. Cause. The upsert of org_plan_assignments failed. Fix. Retry; if persistent, the row may be locked or the unique index is corrupted — escalate to the operator. HTTP 500.