Zenith Tiered Loyalty Platform

Design a loyalty management system for a global retail brand. The system must support multiple tiers (Bronze, Silver, Gold) where users earn points at different rates based on their current tier. Tiers are upgraded or downgraded automatically based on rolling 12-month spending. Additionally, the system must allow for 'Flash Multipliers' where specific categories earn 3x points for a limited time. Design the core logic that handles transaction processing, tier calculation, and points distribution.