infra: add Docker, Compose, and Nginx for staging deployment

- Dockerfile: multi-stage build (deps → builder → runner), standalone output
- docker-compose.yml: app + postgres services, health checks
- nginx/staging.letsbe.biz.conf: reverse proxy ready for certbot SSL
- Updated .env.example with all required secrets

Deploy steps:
1. Copy nginx conf to /etc/nginx/sites-enabled/
2. nginx -t && systemctl reload nginx
3. certbot --nginx -d staging.letsbe.biz
4. Create .env from .env.example
5. docker compose up -d --build

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

.env.example

@@ -1,13 +1,14 @@
-# Database
-DATABASE_URI=postgresql://postgres:postgres@localhost:5432/letsbe
+# ── PostgreSQL ──
+POSTGRES_USER=letsbe
+POSTGRES_PASSWORD=your-secure-postgres-password
 
-# Payload CMS
-PAYLOAD_SECRET=your-secret-key-here-change-in-production
+# ── Payload CMS ──
+PAYLOAD_SECRET=generate-a-random-32-char-secret-here
 
-# OpenRouter (for AI brief generation)
+# ── OpenRouter (AI brief generation) ──
 OPENROUTER_API_KEY=your-openrouter-api-key
 
-# Email (Poste.io SMTP)
+# ── Email (Poste.io SMTP) ──
 SMTP_HOST=mail.letsbe.biz
 SMTP_PORT=587
 SMTP_USER=hello@letsbe.biz
@@ -15,8 +16,8 @@ SMTP_PASS=your-smtp-password
 SMTP_FROM=hello@letsbe.biz
 ADMIN_EMAIL=hello@letsbe.biz
 
-# Cal.com
+# ── Cal.com ──
 NEXT_PUBLIC_CALCOM_URL=https://cal.letsbe.biz
 
-# Site
-NEXT_PUBLIC_SITE_URL=https://letsbe.biz
+# ── Site URL ──
+NEXT_PUBLIC_SITE_URL=https://staging.letsbe.biz