Return active budget alerts and 30d/90d cost trends.

Reads the live cost data for the most recent run, checks whether spend has reached the 80% or 95% alert threshold, and returns trend data computed from .sdd/metrics/cost_history.jsonl.

Both halves of this response are scoped to tenant_id: alerts is computed from the caller's tenant's spend against the caller's tenant's cap, and trend / history_days are narrowed to snapshots recorded for that same tenant. A history snapshot written before per-tenant attribution existed carries no tenant and is excluded from every scoped trend, so a fresh deployment's 30/90-day averages start empty and fill in as new, attributed snapshots accumulate.