fix: resolve port configuration issues and database column mismatch

- Fixed database query in adminDashboard.js using non-existent 'created_at' column
  Changed to use 'scheduled_at' for email_queue table queries
- Updated frontend/.env.example to default to Docker configuration (port 3001/api)
- Clarified DEPLOYMENT_GUIDE.md with separate frontend/backend configuration sections
- Added explicit port configuration warnings to prevent future mismatches
- Added beta features section to README for download protection and deployment script

The 500 errors were caused by:
1. Frontend .env pointing to wrong port (3002 instead of 3001)
2. Database query using 'created_at' instead of 'scheduled_at' for email_queue

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/.env.example

@@ -1,6 +1,9 @@
 # Backend API URL
-# For local development:
-VITE_API_URL=http://localhost:3001
+# For local development with Docker:
+VITE_API_URL=http://localhost:3001/api
+
+# For local development without Docker:
+# VITE_API_URL=http://localhost:3001
 
 # For production behind reverse proxy (Traefik, nginx, etc):
 # VITE_API_URL=/api