Add production deployment guide for GCP Cloud Run

Comprehensive DEPLOYMENT.md covering GCP project setup, split Dockerfiles,
Cloud Run deployment, Cloudflare DNS, and integrations (Clerk, Supabase,
Resend, PostHog, Sentry, Upstash). Includes AI feature gating strategy,
full env var reference, verification checklist, and cost estimates.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2 people