Implement comprehensive security and performance improvements for monitoring dashboard

- Fix WebSocket origin validation to prevent CSRF attacks
- Add input sanitization with HTML escaping to prevent XSS
- Split single RWMutex into 5 specialized locks to reduce contention
- Add configurable memory limits for query log entries
- Implement real-time memory usage tracking and cleanup
- Add new config options: max_query_log_entries, max_memory_mb

Author: jedisct1