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

[nicoalbanese]

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).

[vercel]

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Project	Deployment	Actions	Updated (UTC)
open-agents	Ready	Preview	May 3, 2026 10:27am

[vercel]

Additional Suggestion:

In the /api/models route, getServerSession() throws a NEXT_REDIRECT error for non-allowed managed template users, which is caught by the surrounding try/catch block, producing a 500 error instead of the intended access-denied response.