Add read-only customer AI assistant

[solomon2773]

Summary

• add a customer dashboard assistant grounded in published articles and that customer account only
• exclude credentials, service configuration, payment methods, and other customers
• enforce per-account hourly limits and bounded conversation history
• provide an escalation path that creates a normal support ticket

Validation

• 78 unit tests
• strict TypeScript check
• production Next.js build

Stacked on #25.

Closes #9