chore(devops): docker-compose dev + prod, Dockerfiles, nginx, Makefile

- backend/Dockerfile: Python 3.12 slim, non-root user, WeasyPrint system deps
- backend/Dockerfile.dev: dev variant with hot-reload support
- frontend/Dockerfile: Node 20 alpine build + nginx:alpine serve
- frontend/nginx.conf: SPA routing + /api proxy to backend:8000
- docker-compose.yml: production with healthcheck
- docker-compose.dev.yml: dev with volume mounts and hot-reload
- Makefile: dev, build, up, down, logs, migrate, test, shell, prod targets
- .dockerignore for backend and frontend

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

frontend/.dockerignore

@@ -0,0 +1,7 @@
+node_modules
+dist
+.git
+.gitignore
+*.md
+.env
+.env.*