feat(03-04): Dockerfile SSR multi-stage + docker-compose Traefik port 3000

- Rewrite Dockerfile: node:22-alpine build + runtime, copy .output/, node server
- Add .dockerignore excluding node_modules, .nuxt, .output, src, .git, .planning
- Update docker-compose loadbalancer port from 80 to 3000
- Add SMTP and GA4 environment variables to docker-compose

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

.dockerignore

@@ -0,0 +1,8 @@
+node_modules
+.nuxt
+.output
+dist
+src
+.git
+*.md
+.planning