fix(docker): correct CMD paths for service builds

- backend: node services/backend/dist/index.js (not dist/index.js)
- discord-gateway: node services/discord-gateway/dist/index.js
- frontend: pnpm --filter preview instead of root dev:web (missing frontend/ dir)

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

infra/docker/Dockerfile.frontend

@@ -37,5 +37,5 @@ RUN pnpm --filter './services/frontend' run build
 # Expose port (served by backend)
 EXPOSE 3000
 
-# Start development server
-CMD ["pnpm", "run", "dev:web"]
+# Start frontend (production preview)
+CMD ["pnpm", "--filter", "./services/frontend", "run", "preview", "--", "--host", "0.0.0.0"]