MIKE-04B: recover AWS staging frontend and S3 readiness

From the PR description

MIKE-04B: Recover AWS Staging Frontend and S3 Readiness

Fixes two critical bugs blocking AWS staging.

Bug 1 - Frontend 502/503 (dev mode in ECS)

Root cause: NODE_ENV=development, dev mode image, health check on / not /api/health, NEXT_PUBLIC vars as runtime secrets.

Fix: Dockerfile.prod with build args, postcss.config.mjs, tw-animate-css in deps, task definition rev 4 production mode.

Bug 2 - Backend /ready 503 (storage: false)

Root cause: storage.ts forced static credentials, blocking IAM role.

Fix: Conditional credentials in getClient(), storageEnabled checks bucket name.

Evidence

• Frontend /api/health: 200, /: 200, /login: 200
• Backend /ready: 200 (supabase: true, storage: true)
• Login: 200 (JWT authenticated)
• Both target groups: healthy