feat(deploy): add Docker build + Traefik-based production deployment

- Dockerfile: multi-stage pnpm monorepo build with Next.js standalone output
- .dockerignore: exclude node_modules, .next, .env, dev artifacts
- next.config.ts: enable `output: standalone` for slim runtime image
- compose.prod.yml: replace Caddy with Traefik labels, add migrate init
  service, split internal/external networks

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/next.config.ts

@@ -42,6 +42,7 @@ const securityHeaders = [
 ];
 
 const nextConfig: NextConfig = {
+  output: "standalone",
   async headers() {
     return [
       {