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>

.dockerignore

@@ -0,0 +1,13 @@
+.git
+.gitignore
+node_modules
+**/node_modules
+**/.next
+**/coverage
+**/dist
+*.md
+.env*
+!.env.example
+docker
+.vscode
+.idea