Guardrails on custom metrics

Any event you send can now be a guardrail. Rollout pages show the guardrail history as a timeline. Drafted by Loomvane from the merged pull requests behind v4.12 and edited by a person.

• New — Any event you send can now be a guardrail. Set a threshold and a window; the rollout pauses when the exposed cohort crosses it.

• New — Rollout pages show the guardrail history as a timeline, including who resumed a paused rollout.

• Improved — Flag search now matches on owner initials and tags.

• Fixed — Scheduled steps in a timezone with daylight-saving changes no longer shift by one hour.