Fix critical account deletion, payment, and chat billing bugs

[cursor]

Summary

• delete user-owned data explicitly for admin and self-service account deletion so SQLite/non-cascade deployments do not retain PII
• validate PayPal payment ownership before executing/capturing payments and credit only the authenticated owner
• fail chat turns when the required token ledger charge cannot be recorded, and require sufficient balance for the full platform-token cost

Validation

• python3 -m unittest backend.tests.test_admin_service backend.tests.test_auth_service backend.tests.test_paypal_service backend.tests.test_chat_turn_service