feat: say when the AI budget runs out, on email or any other channel

[AKogut]

The alerting half of #73, on email as asked — and on every other channel, because it goes through the notification path that already exists rather than a new one.

What it fixes

The budget stops root-cause analysis and recorded that in a metric counter. On a self-hosted instance nobody is scraping it, so explanations simply stopped appearing partway through the afternoon and nothing anywhere said why. #288 made the state visible on a page you have to go and look at; this one comes to you.

Subject: [Flakemetry] AI budget spent for today

Root-cause analysis is paused until tomorrow — 200,000 of 200,000 tokens used today.

Spent: 200,000
Budget: 200,000

The message says analysis is paused rather than only reporting numbers. Someone reading this in a mail client has no dashboard open, and "budget exceeded" alone does not tell them anything stopped.

Deduplicated per project per day

The budget is re-checked on every run, so without a key that collapses them a busy afternoon sends one of these per failing suite — which is how a useful alert becomes a filter rule.

My first test proved nothing

It emitted the event on the bus and watched a channel receive it. That tests the routing — bus name, notification type, subscription list — and all of it passed with the emit inside RCA deleted:

$ # emit removed from rca.ts
Tests  1 passed        ← the routing test, still green

Found by deleting the emit, which is the only way that gap shows up. The real test drives processFailures until the budget is exhausted and fails when the emit is gone:

× announces that the budget is spent rather than only counting it
AssertionError: expected [] to have a length of 1

Both tests are kept: one covers the wiring between three pieces that must agree, the other covers the thing that triggers it.

Getting the second one to fire took two attempts — the first used a second failure whose message normalised close enough to the first that it was not a new signature, so the loop skipped it before reaching the budget check. Worth knowing that the budget only applies to signatures that would actually call the model.

62/62 turbo tasks.