♻️ Simplify environment defaults (#2415)

## AI Disclaimer

Using Codex with gpt-5.6-sol. Reviewed manually.

compose.override.yml

@@ -88,15 +88,15 @@ services:
     depends_on:
       - backend
       - mailcatcher
-    env_file:
-      - .env
     environment:
+      - FIRST_SUPERUSER=${FIRST_SUPERUSER?Variable not set}
+      - FIRST_SUPERUSER_PASSWORD=${FIRST_SUPERUSER_PASSWORD?Variable not set}
       - PLAYWRIGHT_BASE_URL=http://backend:8000
       - VITE_API_URL=http://backend:8000
       - MAILCATCHER_HOST=http://mailcatcher:1080
       # For the reports when run locally
       - PLAYWRIGHT_HTML_HOST=0.0.0.0
-      - CI=${CI}
+      - CI=${CI:-}
     volumes:
       - ./frontend/blob-report:/app/frontend/blob-report
       - ./frontend/test-results:/app/frontend/test-results