feat(products): extend product status lifecycle + prelaunch config
- Add 6 product statuses: draft, pre_launch, beta, active, sunset, disabled
- Add PrelaunchConfig with customFields, CAPTCHA, tagline, maxSignups
- Add isValidStatusTransition() helper for safe status changes
- Update getRequestProductId() to block draft/sunset/disabled
- Add getRequestProductIdForPublic() for pre_launch waitlist routes
- Add status transition validation to product update route
- Add PRE_LAUNCH_SIGNUP_SYSTEM.md design doc with full roadmap