fix: proxy /api/* from Vite dev server to the backend

docker-compose.yml

@@ -30,6 +30,11 @@ services:
       - "5173:5173"
     depends_on:
       - app
+    environment:
+      # Vite dev-server proxy target. Inside the compose network, the
+      # backend service is reachable at http://app:8000. The browser
+      # still hits http://localhost:5173/api/v1/* and Vite proxies it.
+      - VITE_API_PROXY_TARGET=http://app:8000
     # Bind-mount the source so Vite HMR sees host edits. node_modules stays
     # inside the container so platform-native deps aren't shadowed.
     volumes:

frontend/vite.config.ts

@@ -1,7 +1,26 @@
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
 
+// `/api/*` calls from the React app are proxied to the FastAPI backend so
+// the dev experience matches a same-origin production deployment. Without
+// this Vite serves `index.html` for `/api/*`, which the browser then tries
+// to JSON.parse — producing "Unexpected token '<'".
+//
+// Override the target via env when needed:
+//   - Local (no Docker): defaults to http://localhost:8000.
+//   - docker-compose:    set VITE_API_PROXY_TARGET=http://app:8000 (inside
+//                        the compose network the backend hostname is `app`).
+const API_PROXY_TARGET = process.env.VITE_API_PROXY_TARGET ?? 'http://localhost:8000'
+
 // https://vite.dev/config/
 export default defineConfig({
   plugins: [react()],
+  server: {
+    proxy: {
+      '/api': {
+        target: API_PROXY_TARGET,
+        changeOrigin: true,
+      },
+    },
+  },
 })