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"]

server.js

@@ -1,63 +1,105 @@
 const express = require('express');
 const path = require('path');
+const { GoogleAuth } = require('google-auth-library');
 
 const app = express();
-const PORT = process.env.PORT || 80;
+const port = process.env.PORT || 8080;
 const BACKEND_URL = process.env.BACKEND_URL;
 
+// Validate required environment variable at startup
+if (!BACKEND_URL) {
+  console.error('ERROR: BACKEND_URL environment variable is required');
+  process.exit(1);
+}
+
+// Initialize Google Auth client (auto-detects credentials on Cloud Run)
+const auth = new GoogleAuth();
+
 app.use(express.json());
 
 // Serve static React build
 app.use(express.static(path.join(__dirname, 'dist')));
 
-// Proxy endpoint - fetches identity token and forwards to backend
+// Proxy endpoint - generates ID token and forwards to backend
 app.post('/api/chat/stream', async (req, res) => {
+  console.log('Proxy request received for /api/chat/stream');
+
   try {
-    // Fetch identity token from GCP metadata server
-    const tokenResponse = await fetch(
-      `http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/identity?audience=${BACKEND_URL}`,
-      { headers: { 'Metadata-Flavor': 'Google' } }
-    );
-    const identityToken = await tokenResponse.text();
+    // Get ID token client with backend URL as audience
+    const client = await auth.getIdTokenClient(BACKEND_URL);
+    const headers = await client.getRequestHeaders();
+
+    console.log('Generated ID token, forwarding to backend...');
 
     // Forward request to backend with auth
     const backendResponse = await fetch(`${BACKEND_URL}/chat/stream`, {
       method: 'POST',
       headers: {
         'Content-Type': 'application/json',
-        'Authorization': `Bearer ${identityToken}`,
+        ...headers,
       },
       body: JSON.stringify(req.body),
     });
 
-    // Stream response back to client
+    // Check if backend returned an error
+    if (!backendResponse.ok) {
+      const errorText = await backendResponse.text();
+      console.error(`Backend error: ${backendResponse.status} - ${errorText}`);
+      res.status(backendResponse.status).json({
+        error: 'Backend error',
+        status: backendResponse.status,
+        message: errorText,
+      });
+      return;
+    }
+
+    // Set SSE headers and flush immediately
     res.setHeader('Content-Type', 'text/event-stream');
     res.setHeader('Cache-Control', 'no-cache');
     res.setHeader('Connection', 'keep-alive');
+    res.flushHeaders?.();
 
+    // Pipe raw SSE stream from backend to browser (no parsing)
     const reader = backendResponse.body.getReader();
-    const pump = async () => {
+
+    // Handle client disconnect
+    req.on('close', () => {
+      console.log('Client disconnected');
+      reader.cancel();
+    });
+
+    // Stream loop
+    while (true) {
       const { done, value } = await reader.read();
       if (done) {
+        console.log('Stream complete');
         res.end();
         return;
       }
       res.write(value);
-      pump();
-    };
-    pump();
-
+    }
   } catch (error) {
     console.error('Proxy error:', error);
-    res.status(500).json({ error: 'Proxy error' });
+    if (!res.headersSent) {
+      res.status(500).json({ error: 'Proxy error', message: error.message });
+    } else {
+      res.end();
+    }
   }
 });
 
+// Health check endpoint for Cloud Run
+app.get('/health', (req, res) => {
+  res.status(200).json({ status: 'healthy' });
+});
+
 // SPA fallback - all other routes serve index.html
 app.get('*', (req, res) => {
   res.sendFile(path.join(__dirname, 'dist', 'index.html'));
 });
 
-app.listen(PORT, () => {
-  console.log(`Server running on port ${PORT}`);
+// Listen on 0.0.0.0 for Cloud Run
+app.listen(port, '0.0.0.0', () => {
+  console.log(`Server listening on ${port}`);
+  console.log(`Backend URL: ${BACKEND_URL}`);
 });