[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.

Start by locating the rollup model and the existing enforcement mechanism referenced in the story, then trace how synthetic spend data is evaluated. Done means configured 80%/90%/100% thresholds trigger webhook and email alerts once per crossing, with deduplication and rate-limiting covered by tests.

由索引模型根据 Issue 内容生成。