Design the multi-tenant architecture for Platform Mastery AI Agents:

Requirements:
- Each customer gets their own agent with private conversation history
- Agents share the same platform KB (Telnyx KB is shared, not per-customer)
- Customer-specific context (their API keys, their account config, their preferences)
- Usage metering and billing per customer
- Role-based access (admin, user, viewer)
- SSO integration

Database schema:
- tenants, users, conversations, messages, api_keys, usage_events, subscriptions
- Platform KB is SHARED (one Telnyx KB serves all Telnyx customers)
- Customer context is PRIVATE (their specific configuration, history)

Security:
- Row-level security in PostgreSQL
- API key encryption
- Audit logging
- Data isolation guarantees