[Story]: Real-time budget-threshold alerting

## Summary
Add real-time budget-threshold alerting so a BU or platform admin is notified as spend approaches a limit, before the budget is exhausted — distinct from historical/after-the-fact reporting.

## Intent / Source of truth
Buyers explicitly distinguish proactive threshold alerting from historical reporting; a chargeback report that only shows what already happened doesn't let a BU owner intervene in time. Part of [Epic] Per-BU cost attribution and chargeback reporting.

## Scope
- [ ] Configurable threshold rules (e.g. 80%/90%/100% of BU or team budget)
- [ ] Alert delivery (webhook + email at minimum)
- [ ] Alert deduplication/rate-limiting so a hovering-near-threshold spend doesn't spam

## Out of scope
- Automatic spend throttling on alert (that's the existing enforcement mechanism's job, not this story's)

## Verification
Test asserting an alert fires exactly once when spend crosses each configured threshold, using synthetic spend data against the rollup model.

## Risk assessment
Alert fatigue from a poorly-tuned deduplication window would train admins to ignore alerts; the story should default to sane thresholds and be configurable per customer.

## AI Usage Declaration
Drafted with AI assistance during the 2026-08-13 cross-repo backlog consolidation and enterprise-readiness research. A human owns intent, verification and consequences.