fix: Add proxy network to staging container

- Add external proxy network to portfolio-staging service
- Ensures staging container can communicate with reverse proxy
- Matches production configuration

docker-compose.staging.yml

@@ -26,6 +26,7 @@ services:
       - portfolio_staging_data:/app/.next/cache
     networks:
       - portfolio_staging_net
+      - proxy
     depends_on:
       postgres-staging:
         condition: service_healthy
@@ -112,3 +113,5 @@ volumes:
 networks:
   portfolio_staging_net:
     driver: bridge
+  proxy:
+    external: true