Remove a group suppression

Soft-deletes all active blocks for (account, group, normalized email) — one removed audit event per block (actor: manual). The email path segment is normalized (trim + lower-case) before matching. Idempotent on already-removed rows (returns 404 since they're no longer active).

Two distinct 404 cases: a missing/cross-account group returns 10001 "The requested unsubscribe group was not found"; a group that exists but has no active suppression for that email returns 10001 "The requested group suppression was not found".