feat(deploy): add Railway template for one-click deployment

[AbirAbbas]

Summary

• Add Railway deployment template for easy one-click deployment of the AgentField control plane with PostgreSQL

Changes Made

• railway.toml and railway.json at repo root for Railway auto-detection
• Points to existing deployments/docker/Dockerfile.control-plane (no duplication)
• Configures health check at /api/v1/health
• deployments/railway/README.md with setup instructions and deploy button

How to Use

1. Click "Deploy on Railway" button in the README
2. Add PostgreSQL plugin in Railway
3. Configure AGENTFIELD_POSTGRES_URL=${{Postgres.DATABASE_URL}}
4. Deploy

Test Plan

• Deploy from repo to Railway
• Verify Dockerfile is detected and builds correctly
• Verify health check works
• Verify PostgreSQL connection with auto-migrations

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