ci: kaniko caching, casecomp-api image rename, optimized Dockerfile

[Pyronewbic]

Summary

• Kaniko builder with 7-day layer cache for Cloud Build
• Dockerfile: playwright deps before npm install for better layer caching
• Docker image renamed from cardscrapebot to casecomp-api
• Tests run on push to dev/main + PRs to main
• Removed old cloudbuild.yaml

Test plan

• cloudbuild.yml uses kaniko with --cache=true
• Dockerfile layers ordered: base → playwright → npm → code
• Image name updated in deploy.yml, variables.tf, package.json
• Tests pass
• Deploy succeeds on merge