ci: add production migration deployment flow

[linuka7]

Milestone 19 — Deployment + CI/CD — Batch 2

Adds the production deployment flow for Railway.

Changes

• explicit --migrate application mode for controlled EF Core production migrations
• Railway deployment configuration using the root Dockerfile
• pre-deploy migration command before traffic switch
• readiness health check on /api/health/ready
• Railway deployment documentation and smoke-test guidance

Safety

• normal API startup still does not auto-apply migrations
• migration failure blocks deployment
• production secrets remain external to Git

Verification

• solution build passed locally
• explicit migration command smoke test passed locally
• full regression suite passed locally
• Railway config validation passed
• git diff checks passed