feat(deploy): add production runtime foundation

docs/deployment-production.md

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+# Production Deployment Runtime Foundation
+
+This project can be deployed in a dedicated Proxmox container using Docker Compose. The files added here establish a minimal production runtime baseline without changing application business logic or Supabase security boundaries.
+
+## Dockerfile purpose
+
+The `Dockerfile` uses a multi-stage build on Node 20:
+
+- installs dependencies
+- runs `next build`
+- prepares a lean runtime image with production dependencies
+- runs the app with `next start` on port `3000`
+
+This keeps the runtime image small and focused on serving the built Next.js app.
+
+## docker-compose.prod.yml purpose
+
+`docker-compose.prod.yml` defines one service:
+
+- service name: `web`
+- container name: `plesk-agency-portal`
+- restart policy: `unless-stopped`
+- environment from `.env`
+- port mapping `3000:3000`
+- health check against `/api/health`
+
+## External Supabase requirement
+
+Supabase remains external to this application container. The app container must only connect to Supabase via environment variables. No local Supabase container is included in this production compose file.
+
+## Health endpoint usage
+
+`GET /api/health` provides a fast, dependency-light runtime check for container orchestration and uptime checks.
+
+Example:
+
+```bash
+curl -s http://localhost:3000/api/health
+```
+
+Expected response shape:
+
+- `ok: true`
+- `service: "plesk-agency-portal"`
+- `timestamp`
+- `environment`
+
+## Production environment expectations
+
+Before deploying, set production-safe values in `.env` (using `.env.example` as the template), including:
+
+- app URL and `NODE_ENV`
+- Supabase public and server credentials
+- Plesk credential encryption key
+- Stripe secrets
+- cron/job shared secrets
+
+Keep secrets out of source control.