chore: casecomp-site Cloud Run + CDN, READMEs, CHANGELOG

[Pyronewbic]

Summary

• Cloud Run casecomp-site for frontend SSR with Cloud CDN
• Terraform: site service, NEG, backend service with CDN policy, SSL cert
• URL map routes by host: casecomp.xyz → site, api.casecomp.xyz → API
• Cloud Run renamed from cardscrapebot to casecomp-api
• Sandbox API key (CC_LIVE_SANDBOX_) with 5/min rate limit
• Per-subgrade AI grading, security hardening, OAuth warmup
• READMEs and CHANGELOG updated

Manual verification

• https://casecomp.xyz serves frontend (pending SSL cert)
• https://api.casecomp.xyz/api/health returns ok