subscription_exists
POST /v1/subscription on a tenant that already has one (uq_subscriptions_tenant_id). A plan change amends the existing subscription; it never opens a second. Also answered when a concurrent request won the constraint.
Problem shape
Every non-2xx response is an RFC 9457 application/problem+json body. The type URI below is stable and machine-matchable, so branch on the final path segment (the code) and treat the status as redundant confirmation. See the error-handling guide.
{
"type": "https://docs.vocapable.com/errors/subscription_exists",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}