4.2 Budget Threshold Events via Options Flow

**Effort:** M
**What:** Let users configure percentage thresholds (50%, 75%, 90%, 100%) per category or globally. Fire events when thresholds are crossed.
**Implementation:** Store previous percentage_used, compare on update, fire events on threshold crossings.
**🔗 Riding on:** Same "compare previous vs current" infrastructure as 4.1.