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>

backend/src/routes/adminDashboard.js

@@ -166,7 +166,7 @@ router.get('/health', adminAuth, async (req, res) => {
     
     const [failedEmails] = await db('email_queue')
       .where('status', 'failed')
-      .where('created_at', '>=', twentyFourHoursAgo.toISOString())
+      .where('scheduled_at', '>=', twentyFourHoursAgo.toISOString())
       .count('* as count');
 
     const emailStatus = failedEmails.count > 10 ? 'warning' : 'healthy';