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>

apps/web/src/lib/api.ts

@@ -5,7 +5,7 @@
  * All functions accept a cookie string for server-side auth forwarding.
  */
 
-const API_URL = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:3001';
+const API_URL = process.env.API_INTERNAL_URL || process.env.NEXT_PUBLIC_API_URL || 'http://localhost:3001';
 
 /**
  * Module data shape from the API.