Fix SSE 500: move API rewrites to afterFiles so route handlers take priority

The flat-array form of rewrites() runs as beforeFiles, intercepting /api/v1/*
requests before Next.js can match route handlers. This caused the SSE stream
endpoints (outlines/stream, presentation/stream, jobs/stream) to be handled
by the buffering HTTP proxy instead of the custom streaming route handlers,
resulting in ECONNRESET → 500 on the browser.

afterFiles rewrites only apply when no matching app/api/ file exists, so
route handlers now win over the backend proxy for all /api/v1/* paths that
have explicit handlers.

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

frontend/next.config.mjs

@@ -6,22 +6,30 @@ const nextConfig = {
   distDir: ".next-build",
 
 
-  // Proxy API and static asset requests to FastAPI backend
+  // Proxy API and static asset requests to FastAPI backend.
+  // afterFiles: Next.js checks route handlers first; only falls through to
+  // these rewrites if no matching app/api/ route handler exists.
+  // This ensures SSE route handlers (outlines/stream, presentation/stream, etc.)
+  // are served directly without being intercepted by the catch-all /api/v1/* rewrite.
   async rewrites() {
-    return [
-      {
-        source: '/api/v1/:path*',
-        destination: `${API_URL}/api/v1/:path*`,
-      },
-      {
-        source: '/app_data/:path*',
-        destination: `${API_URL}/app_data/:path*`,
-      },
-      {
-        source: '/static/:path*',
-        destination: `${API_URL}/static/:path*`,
-      },
-    ];
+    return {
+      beforeFiles: [],
+      afterFiles: [
+        {
+          source: '/api/v1/:path*',
+          destination: `${API_URL}/api/v1/:path*`,
+        },
+        {
+          source: '/app_data/:path*',
+          destination: `${API_URL}/app_data/:path*`,
+        },
+        {
+          source: '/static/:path*',
+          destination: `${API_URL}/static/:path*`,
+        },
+      ],
+      fallback: [],
+    };
   },
 
   images: {