fix: use external network for dev compose to avoid label conflicts

The portfolio_dev network was created manually by the pipeline, causing
docker-compose to fail with label mismatch errors. Now:
- Network is marked as external in compose (compose doesn't try to own it)
- Network creation moved before compose up in the pipeline
- Redundant network check later in pipeline removed

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

docker-compose.dev.minimal.yml

@@ -36,8 +36,7 @@ services:
 
 networks:
   portfolio_dev:
-    name: portfolio_dev
-    driver: bridge
+    external: true
 
 volumes:
   postgres_dev_data: