Support ticket routing and SLA

Design a support-ticket routing + SLA-tracking system for an IT helpdesk handling ~40K tickets/day across ~800 agents in multiple regions/skills. Requirements: route incoming tickets to the right agent queue by skill/priority/region, track SLA timers (first-response and resolution targets that vary by priority and pause during business-hours-off / customer-waiting states), escalate or alert when an SLA is about to breach, and rebalance load when an agent goes offline. Read-heavy dashboards but timer correctness is critical.