fix: Dockerfile prisma generation, migrations entrypoint, and health endpoint 503

[Levi-Ojukwu]

Summary

• Dockerfile: prisma generate not run before nest build — production image may lack generated Prisma client bindings #68: Run prisma generate before nest build in Dockerfile and copy generated
  client bindings to the production stage so clean builds succeed without cached layers.
• Dockerfile: CMD does not run prisma migrate deploy — database schema not applied when container starts #69: Add entrypoint.sh that runs prisma migrate deploy before starting the app.
  Container exits non-zero if migrations fail. Idempotent on already-migrated databases.
• health.controller.ts: returns HTTP 200 with status 'degraded' when DB is unreachable — load balancers cannot detect outage #67: Health endpoint now returns HTTP 503 (SERVICE_UNAVAILABLE) instead of 200 when
  the database is unreachable, enabling load balancers and k8s probes to detect outages.
  Response body still includes structured JSON for debugging.
• docker-compose.yml: Redis service defined and health-checked but no Redis client installed — inconsistent dependency #70: Skipped — Redis IS actively used for distributed rate limiting via
  ThrottlerStorageRedisService in app.module.ts. The docker-compose Redis service is
  required.

closes #67
closes #68
closes #69
closes #70

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