feat: implement server-side proxy with google-auth-library

Rewrite server.js to use google-auth-library for generating Google ID
tokens when authenticating requests to the backend Cloud Run service.

Key changes:
- Replace metadata server token fetch with GoogleAuth client
- Implement proper SSE streaming with raw byte passthrough
- Add health check endpoint for Cloud Run monitoring
- Update server to listen on 0.0.0.0:8080 for Cloud Run compatibility
- Add environment variable validation at startup
- Improve error handling and logging throughout request lifecycle

Update Dockerfile to install google-auth-library dependency and expose
port 8080 instead of port 80.

Dockerfile

@@ -11,6 +11,6 @@ FROM node:20-alpine
 WORKDIR /app
 COPY --from=build /app/dist ./dist
 COPY server.js .
-RUN npm install express@4
-EXPOSE 80
+RUN npm install express@4 google-auth-library
+EXPOSE 8080
 CMD ["node", "server.js"]