Self-serve account deletion.

Cancels the Stripe subscription immediately (if present), then purges all org data in one transaction: usage_daily → usage_monthly → api_keys → organizations. Does NOT delete auth.users — the web BFF does that with the service role. Does NOT touch webhook_events (global Stripe idempotency table).

Abort policy: if Stripe cancel fails for any reason other than "already gone", returns 502 and deletes NOTHING (never orphan an active subscription).