fix: critical database connection pool exhaustion issues

- Disabled duplicate email service (emailService.js) that was creating redundant connections
- Increased connection pool size from 10 to 25 for production environment
- Extended session timeout cache from 5 to 30 minutes to reduce DB queries
- Added connection retry logic with exponential backoff for transient failures
- Fixed password validation to use retry wrapper and correct setting key
- Updated public settings and gallery middleware to handle connection failures gracefully

These changes address the "Connection terminated unexpectedly" errors in production by:
1. Reducing unnecessary database connections
2. Increasing available connection pool capacity
3. Implementing automatic retry for transient connection failures
4. Caching frequently accessed data for longer periods

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

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

backend/src/services/emailService.js

@@ -60,6 +60,7 @@ async function processEmailQueue() {
 }
 
 // Start email queue processor
-setInterval(processEmailQueue, 60000); // Process every minute
+// DISABLED: Using emailProcessor.js instead to prevent duplicate connections
+// setInterval(processEmailQueue, 60000); // Process every minute
 
 module.exports = { sendEmail, processEmailQueue };