Fix permission error when writing admin credentials

- Changed credential file location from /app/ to /app/data/
- Added directory creation with recursive flag
- Updated console messages to show correct file location
- The data/ directory is already owned by nodejs user in Dockerfile

The error occurred because the nodejs user doesn't have write
permission to /app/ directory, but does have permission to /app/data/
which is explicitly created and chowned in the Dockerfile.

DEPLOYMENT_GUIDE.md

@@ -180,7 +180,7 @@ docker-compose up -d
 # Run database migrations
 docker-compose exec backend npm run migrate
 
-# Admin credentials will be displayed and saved to ADMIN_CREDENTIALS.txt
+# Admin credentials will be displayed and saved to /data/ADMIN_CREDENTIALS.txt
 ```
 
 #### Step 4: Configure Nginx