Implement session cookie isolation between Django apps

- Added configurable SESSION_COOKIE_NAME and CSRF_COOKIE_NAME to Django settings
- Main app now uses 'stiftung_sessionid' instead of default 'sessionid'
- Paperless continues using default 'sessionid' for separation
- All configuration centralized in .env files as requested
- Updated both development and production compose configurations
- Added session settings to env templates for easy deployment

This resolves the session conflict where logging into one app would
kick out sessions from the other app. Both applications now maintain
independent login sessions.

compose.yml

@@ -43,6 +43,8 @@ services:
       - LANGUAGE_CODE=${LANGUAGE_CODE}
       - TIME_ZONE=${TIME_ZONE}
       - REDIS_URL=${REDIS_URL}
+      - SESSION_COOKIE_NAME=${SESSION_COOKIE_NAME}
+      - CSRF_COOKIE_NAME=${CSRF_COOKIE_NAME}
       - PAPERLESS_API_URL=${PAPERLESS_API_URL}
       - PAPERLESS_API_TOKEN=${PAPERLESS_API_TOKEN}
       - PAPERLESS_REQUIRED_TAG=${PAPERLESS_REQUIRED_TAG}