docs: create deployment security hardening guide

[salishforge]

Summary

No comprehensive deployment hardening guide exists. Operators need guidance on:

1. Required environment variables for production (AUDIT_HMAC_KEY, ADMIN_TOKEN, OAUTH2 config)
2. PostgreSQL hardening (RLS, connection limits, TLS)
3. Redis hardening (AUTH, TLS, network isolation)
4. HTTPS enforcement and reverse proxy configuration
5. Network segmentation (LLM providers, database, Redis)
6. Backup and disaster recovery for memory data
7. Monitoring and alerting on security events

Deliverable

Create DEPLOYMENT-SECURITY.md with production checklist and reference architecture.