fix(web): proxy client API calls through Next.js to fix production connectivity

NEXT_PUBLIC_API_URL was undefined at build time, causing client bundles to
fall back to http://localhost:3001 — unreachable from the browser in prod.

- Add /api-proxy rewrite in next.config.ts (forwards to API_INTERNAL_URL at runtime)
- Bake NEXT_PUBLIC_API_URL=/api-proxy at build time in Dockerfile
- Fix api.ts to prefer API_INTERNAL_URL for server-side calls
- Fix docker-compose.prod.yml: set NEXT_PUBLIC_API_URL=http://api:3001 for runtime server-side code

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

docker-compose.prod.yml

@@ -6,7 +6,7 @@ services:
       - "3000:3000"
     environment:
       HOSTNAME: "0.0.0.0"
-      NEXT_PUBLIC_API_URL: ${APP_URL:-http://localhost:3001}
+      NEXT_PUBLIC_API_URL: http://api:3001
       API_INTERNAL_URL: "http://api:3001"
       JWT_SECRET: ${JWT_SECRET}
     networks: