feat!: backend-only architecture, remove frontend/nginx (v1.2.0)

- Remove frontend/ and nginx/ from core (each customer owns their frontend)
- Update docker-compose.yml to backend-only (postgres + redis + backend)
- Update docker-compose.prod.yml to backend-only
- Add CLAUDE.md with dev guidelines
- Update README.md with new architecture diagram

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

VERSION

@@ -1 +1 @@
-1.1.0
+1.2.0