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>

docker-compose.yml

@@ -0,0 +1,45 @@
+services:
+  db:
+    image: postgres:16-alpine
+    restart: unless-stopped
+    environment:
+      POSTGRES_DB: letsbe
+      POSTGRES_USER: ${POSTGRES_USER:-letsbe}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:?Set POSTGRES_PASSWORD in .env}
+    volumes:
+      - pgdata:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-letsbe} -d letsbe"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+
+  app:
+    build:
+      context: .
+      args:
+        DATABASE_URI: postgresql://${POSTGRES_USER:-letsbe}:${POSTGRES_PASSWORD}@db:5432/letsbe
+        PAYLOAD_SECRET: ${PAYLOAD_SECRET:?Set PAYLOAD_SECRET in .env}
+        NEXT_PUBLIC_SITE_URL: ${NEXT_PUBLIC_SITE_URL:-https://staging.letsbe.biz}
+        NEXT_PUBLIC_CALCOM_URL: ${NEXT_PUBLIC_CALCOM_URL:-}
+    restart: unless-stopped
+    ports:
+      - "127.0.0.1:3000:3000"
+    environment:
+      DATABASE_URI: postgresql://${POSTGRES_USER:-letsbe}:${POSTGRES_PASSWORD}@db:5432/letsbe
+      PAYLOAD_SECRET: ${PAYLOAD_SECRET}
+      NEXT_PUBLIC_SITE_URL: ${NEXT_PUBLIC_SITE_URL:-https://staging.letsbe.biz}
+      NEXT_PUBLIC_CALCOM_URL: ${NEXT_PUBLIC_CALCOM_URL:-}
+      SMTP_HOST: ${SMTP_HOST:-}
+      SMTP_PORT: ${SMTP_PORT:-587}
+      SMTP_USER: ${SMTP_USER:-}
+      SMTP_PASS: ${SMTP_PASS:-}
+      SMTP_FROM: ${SMTP_FROM:-hello@letsbe.biz}
+      ADMIN_EMAIL: ${ADMIN_EMAIL:-hello@letsbe.biz}
+      OPENROUTER_API_KEY: ${OPENROUTER_API_KEY:-}
+    depends_on:
+      db:
+        condition: service_healthy
+
+volumes:
+  pgdata: