Fix Railway ECONNREFUSED error by unifying backend URL resolution

Problem: Next.js frontend in production mode falls back to http://backend:8000
(Docker Compose internal hostname) when BACKEND_URL env var is not set.
In Railway's distributed environment, this hostname doesn't exist, causing
ECONNREFUSED errors.

Solution: Create unified getBackendUrl() function with consistent fallback
priority across all server-side proxying files:
1. Explicit BACKEND_URL (for Railway / custom deployment)
2. Development mode -> http://localhost:8000
3. NEXT_PUBLIC_API_URL (may be set in Railway)
4. Docker Compose default -> http://backend:8000

Changes:
- New: frontend/lib/backend-url.ts - centralized URL resolution
- Updated: frontend/next.config.ts - use getBackendUrl()
- Updated: frontend/app/api/chat/route.ts - use getBackendUrl()
- Updated: frontend/app/api/auth/google/token/route.ts - use getBackendUrl()

Railway users must set BACKEND_URL=https://<backend-service>.up.railway.app

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/app/api/auth/google/token/route.ts

@@ -3,14 +3,11 @@
  * This proxies the token exchange to the backend
  */
 import { NextRequest, NextResponse } from "next/server";
-
-// Use backend internal URL if available (Railway), fallback to public URL
-const BACKEND_URL = process.env.BACKEND_URL || 
-  process.env.NEXT_PUBLIC_API_URL || 
-  "http://localhost:8000";
+import { getBackendUrl } from "@/lib/backend-url";
 
 export async function POST(request: NextRequest) {
   try {
+    const backendUrl = getBackendUrl();
     const body = await request.json();
     const { code, redirect_uri } = body;
 
@@ -22,7 +19,7 @@ export async function POST(request: NextRequest) {
     }
 
     // Forward to backend
-    const backendResponse = await fetch(`${BACKEND_URL}/api/auth/google/token`, {
+    const backendResponse = await fetch(`${backendUrl}/api/auth/google/token`, {
       method: "POST",
       headers: {
         "Content-Type": "application/json",

frontend/app/api/chat/route.ts

@@ -1,11 +1,9 @@
 import { NextResponse } from "next/server";
+import { getBackendUrl } from "@/lib/backend-url";
 
 export async function POST(req: Request) {
   try {
-    const isDev = process.env.NODE_ENV === "development";
-    const backendUrl =
-      process.env.BACKEND_URL ||
-      (isDev ? "http://localhost:8000" : "http://backend:8000");
+    const backendUrl = getBackendUrl();
 
     // Read the complete body from the request
     const bodyText = await req.text();

frontend/lib/backend-url.ts

@@ -0,0 +1,27 @@
+/**
+ * Unified backend URL resolution for server-side proxying.
+ *
+ * Priority:
+ *  1. BACKEND_URL          – explicit override (Railway / custom deployment)
+ *  2. Development mode     – http://localhost:8000
+ *  3. NEXT_PUBLIC_API_URL  – fallback (often set in Railway for client-side too)
+ *  4. Docker Compose default – http://backend:8000
+ *
+ * Used by next.config.ts rewrites and Next.js API route handlers.
+ */
+export function getBackendUrl(): string {
+  if (process.env.BACKEND_URL) {
+    return process.env.BACKEND_URL;
+  }
+
+  if (process.env.NODE_ENV === "development") {
+    return "http://localhost:8000";
+  }
+
+  if (process.env.NEXT_PUBLIC_API_URL) {
+    return process.env.NEXT_PUBLIC_API_URL;
+  }
+
+  // Docker Compose internal hostname (only works when both services share a Docker network)
+  return "http://backend:8000";
+}

frontend/next.config.ts

@@ -1,4 +1,5 @@
 import type { NextConfig } from "next";
+import { getBackendUrl } from "./lib/backend-url";
 
 const nextConfig: NextConfig = {
   output: 'standalone',
@@ -56,13 +57,7 @@ const nextConfig: NextConfig = {
   },
   
   async rewrites() {
-    // In development: use localhost backend
-    // In production (Railway): use BACKEND_URL env var or fallback to Railway URL
-    const isDev = process.env.NODE_ENV === 'development';
-    // Default to http://backend:8000 in production (Docker) if not set
-    const backendUrl = process.env.BACKEND_URL || 
-      (isDev ? "http://localhost:8000" : "http://backend:8000");
-    
+    const backendUrl = getBackendUrl();
     console.log(`[Next.js] Rewriting API requests to: ${backendUrl}`);
     
     return [