Downgrade recovered webhook backlog alert

[Phloraxx]

Summary

• keep historical exhausted webhook deliveries visible
• downgrade the aggregate alert from critical to warning once a newer webhook delivery proves transport recovery
• preserve critical severity while transport has not recovered
• preserve alert resolution once no exhausted deliveries remain
• do not retry, delete, or mutate webhook delivery rows

Production evidence

The verified offline 2026-08-31 03:00 UTC backup contains 170 exhausted deliveries, all HTTP 404 from Aug 12–13, while newer deliveries to the same destination host succeeded with HTTP 200 on Aug 29–30. The current alert already records transportRecovered=true; this PR makes severity reflect that state.

Validation

• focused TestWebhookExhaustionIsOneAggregateCondition passes
• frontend typecheck/build passes
• go test -count=1 ./... passes
• git diff --check passes

No payment matching, webhook delivery/retry behavior, database schema, or customer checkout behavior is changed.