feat(api): add an alert evaluator extension point

.changeset/alert-evaluator-extension-point.md

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+---
+'@hyperdx/api': patch
+---
+
+feat: add an internal extension point for alert detection strategies; threshold alerts are evaluated as before, and the `alerts.evaluate` metric now carries an `alert_evaluator` attribute

packages/api/src/extensions/index.ts

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+// Registers optional extensions with the API's extension points, such as alert
+// evaluators (see tasks/checkAlerts/evaluators.ts). Extension points call this
+// once, on first use, so it runs after every module has loaded. This build
+// registers none.
+export const registerExtensions = (): void => {};