feat: gate hosted access by email domain instead of trial limits

Summary

• Replaces complex "trial" restrictions with strict email domain gating for hosted deployments.
• Users without an approved email domain (configurable, defaults to @vercel.com) are redirected to the /deploy-your-own page.
• Secures backend APIs to return 403 Access Denied for unauthorized domains instead of enforcing trial limits.
• Removes previous trial limitations (message limits, session limits, code editor restrictions, model hiding).