docs: clarify domain registration status and DNS configuration for problemspot.tech #2

Summary

Clarify and document the registration status of the proposed domain problemspot.tech across project documentation, and track DNS configuration for web deployment.

Background

• In README.md (line 18) and docs/PRD.md (line 7), problemspot.tech was proposed with status (availability not verified).
• Live WHOIS lookups show problemspot.tech was registered on 2026-10-13 via Namify Domains Inc (Tucows/Radix).

Acceptance Criteria

• Confirm domain ownership with project maintainers.
• Update README.md and docs/PRD.md to reflect verified domain status.
• Document DNS configuration records and deployment target (e.g., Vercel, Supabase custom domain, Cloudflare).