fix: allow TRANSAK_ENV to be set via Infisical (default STAGING)

Hardcoded PRODUCTION overrode the Infisical value. Use env var with
STAGING default until Transak production gateway auth issue is resolved.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -60,7 +60,7 @@ services:
       - NOTEBOOK_API_URL=http://open-notebook:5055
       - SPLIT_360_URL=http://video360-splitter:5000
       - SCRIBUS_BRIDGE_URL=http://scribus-novnc:8765
-      - TRANSAK_ENV=PRODUCTION
+      - TRANSAK_ENV=${TRANSAK_ENV:-STAGING}
       - TRANSAK_API_KEY_PRODUCTION=${TRANSAK_API_KEY_PRODUCTION}
       - SCRIBUS_BRIDGE_SECRET=${SCRIBUS_BRIDGE_SECRET}
       - SCRIBUS_NOVNC_URL=https://design.rspace.online