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>

Dockerfile

@@ -0,0 +1,49 @@
+FROM node:20-alpine AS base
+
+# ── Dependencies ──
+FROM base AS deps
+WORKDIR /app
+COPY package.json package-lock.json ./
+RUN npm ci --ignore-scripts
+RUN npm rebuild sharp
+
+# ── Builder ──
+FROM base AS builder
+WORKDIR /app
+COPY --from=deps /app/node_modules ./node_modules
+COPY . .
+
+# Build args become env vars at build time
+ARG DATABASE_URI
+ARG PAYLOAD_SECRET
+ARG NEXT_PUBLIC_SITE_URL
+ARG NEXT_PUBLIC_CALCOM_URL
+
+ENV DATABASE_URI=${DATABASE_URI}
+ENV PAYLOAD_SECRET=${PAYLOAD_SECRET}
+ENV NEXT_PUBLIC_SITE_URL=${NEXT_PUBLIC_SITE_URL}
+ENV NEXT_PUBLIC_CALCOM_URL=${NEXT_PUBLIC_CALCOM_URL}
+
+RUN npm run build
+
+# ── Runner ──
+FROM base AS runner
+WORKDIR /app
+
+ENV NODE_ENV=production
+ENV HOSTNAME=0.0.0.0
+ENV PORT=3000
+
+RUN addgroup --system --gid 1001 nodejs
+RUN adduser --system --uid 1001 nextjs
+
+# Copy standalone output
+COPY --from=builder /app/.next/standalone ./
+COPY --from=builder /app/.next/static ./.next/static
+COPY --from=builder /app/public ./public
+
+USER nextjs
+
+EXPOSE 3000
+
+CMD ["node", "server.js"]