Add production environment update instructions

- Created PRODUCTION_ENV_UPDATE.md with step-by-step fix for CookieError
- Provides SSH commands to add missing SESSION_COOKIE_NAME and CSRF_COOKIE_NAME
- Includes verification steps to confirm fix works

PRODUCTION_ENV_UPDATE.md

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+# URGENT: Production Environment Update Required
+
+## Issue
+The production site is experiencing a CookieError due to missing session cookie configuration.
+
+## Fix Required
+SSH into the production server and add these lines to the `/opt/stiftung/.env` file:
+
+```bash
+# SSH into production server
+ssh user@vhtv-stiftung.de
+
+# Navigate to stiftung directory  
+cd /opt/stiftung
+
+# Edit the .env file
+nano .env
+
+# Add these lines to the .env file:
+SESSION_COOKIE_NAME=stiftung_sessionid
+CSRF_COOKIE_NAME=stiftung_csrftoken
+
+# Save and exit (Ctrl+X, Y, Enter)
+
+# Restart the containers to apply changes
+docker-compose -f compose.yml restart web worker beat
+```
+
+## Background
+- These new environment variables prevent session conflicts between the main Stiftung app and Paperless
+- Without them, Django tries to set cookies with empty names, causing the CookieError
+- The fix is already deployed in the code, but the environment variables need to be manually added to production
+
+## Verification
+After adding the variables and restarting:
+- Visit https://vhtv-stiftung.de/login/ 
+- Login should work without CookieError
+- Both Stiftung and Paperless apps should maintain separate sessions