Add arkose anomaly detection module

This MR introduces Arkose anomaly detection by evaluating the current bucket’s token verification rate against a rolling baseline sourced from the Redis vrate stream. When the current rate deviates significantly from historical behavior, the system can surface an anomaly signal that downstream fail-open logic will use in future MRs.

NOTE: This MR needs to come after this one: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/211653