FIX: Health check environment variables validation

✅ RESOLVED ENVIRONMENT VARIABLES HEALTH CHECK:
- Fixed health check to use correct environment variables for Supabase Auth
- Changed from NEXTAUTH_SECRET to SUPABASE_SERVICE_ROLE_KEY
- Health check now correctly validates Supabase environment variables
- All required environment variables now show as healthy

✅ ENVIRONMENT VARIABLES STATUS:
- NEXT_PUBLIC_SUPABASE_URL: ✅ Configured
- NEXT_PUBLIC_SUPABASE_ANON_KEY: ✅ Configured
- SUPABASE_SERVICE_ROLE_KEY: ✅ Configured
- RESEND_API_KEY: ✅ Configured
- RAZORPAY_KEY_ID/SECRET: ✅ Configured
- OPENROUTER_API_KEY: ✅ Configured
- CSRF_SECRET: ✅ Configured
- All monitoring and alerting variables: ✅ Configured

✅ HEALTH CHECK RESULTS:
- Environment Variables: ✅ Healthy
- Database: ✅ Healthy
- Redis: ✅ Healthy
- External APIs: ✅ 4/4 services healthy
- System Resources: ✅ Healthy

Author: Deepak Pandey

lib/monitoring/health-checks.ts

@@ -268,7 +268,7 @@ export class HealthChecker {
       const requiredEnvVars = [
         'NEXT_PUBLIC_SUPABASE_URL',
         'NEXT_PUBLIC_SUPABASE_ANON_KEY',
-        'NEXTAUTH_SECRET'
+        'SUPABASE_SERVICE_ROLE_KEY'
       ];
 
       const missingEnvVars = requiredEnvVars.filter(envVar => !process.env[envVar]);