DELETE /api/organizations now cascade-deletes the sole-org requester (personal org)

What this changes for operators

• Operators must understand that deleting a personal org now also deletes the requesting user account, enabling re-onboarding on next login — a destructive identity-state change behind one endpoint.
• Changes operating-state semantics of an existing destructive API: requires backup discipline before org deletion; multi-org members are protected by preflight orphan detection.
• Verification path: test DELETE /api/organizations against a sole-org account vs a multi-org member and confirm orphan-rejection behavior.