[codex] Add shared serverless Arcade API

[JeremyDemers]

What changed

• Adds a shared FastAPI/Lambda backend for both arcade games.
• Adds Google credential verification, signed sessions, DynamoDB persistence, API Gateway, and Secrets Manager integration.
• Restricts production CORS origins and adds build/deploy scripts, tests, documentation, and CI.

Why

The games previously depended on separate local APIs. This gives the integrated portfolio one production serverless API with persistent scores and authentication.

Impact

The deployed Tetris and Neon Shatter clients can share authentication and game data through the same AWS endpoint.

Validation

• Serverless test suite: 5 passed
• Lambda deployment package built successfully
• Terraform validation passed
• Production Terraform drift check: no changes
• Live /health and production CORS preflight passed