Add deployment documentation and production runbook

[kivo360]

Problem

The project has production URLs (omoios.dev, api.omoios.dev) and Railway deployment, but there's no deployment documentation covering:

• How to deploy to Railway
• Environment variables needed for production
• Database migration strategy for production
• Rollback procedures
• Monitoring and alerting setup

What to do

Create docs/deployment/ with:

1. railway-deployment.md: Step-by-step Railway setup, service configuration, environment variables
2. production-runbook.md: Common operations (deploy, rollback, scale, debug), health check commands, log access
3. environment-variables.md: Complete list of required env vars for each service

Difficulty: Easy-Medium | Skill: Docs (DevOps knowledge helpful)