Feature request: one-directional anomaly detection alerts (high-only / low-only)

Feature request

Summary

Let anomaly-detection alerts fire in a single direction: anomalously high or anomalously low, instead of always firing on any large deviation in either direction.

Problem

Someone with an anomaly-detection alert on a conversion-rate metric may only care when it drops, but the alert also fires when the value is anomalously high: noise they don't want to be paged on. PostHog's anomaly detectors score on the magnitude of a deviation and discard its sign, so there's no way to say "only alert me when this drops." The only escape today is to abandon the adaptive detector for a fixed threshold, which loses the whole point of anomaly detection (it won't adapt to the baseline, trend, or seasonality).

Proposed

Add a direction option to anomaly-detection alerts: both (default, current behavior), high-only, and low-only. When a direction is set, a point only counts as an anomaly if the sign of its deviation matches. The scoring is unchanged, we just gate the final fire/no-fire decision on direction. The classic fixed-threshold alert path already does this via lower

Start by tracing the anomaly-detection alert evaluation and the classic fixed-threshold alert path, especially its existing lower/upper direction handling. Add the proposed both, high-only, and low-only behavior while leaving anomaly scoring unchanged; done means only deviations matching the selected direction fire, with both preserving current behavior.