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.discord-gateway

@@ -38,4 +38,4 @@ RUN pnpm --filter './services/discord-gateway' run build
 RUN mkdir -p /app/recordings
 
 # Start discord gateway
-CMD ["node", "dist/index.js"]
+CMD ["node", "services/discord-gateway/dist/index.js"]