Pause continuous monitoring for an identity

Pauses continuous AML monitoring on this person. Monitoring is a person-level state — pause/resume operates on the identity, not on individual screening checks. Sets `monitoring_paused_reason='manual'` and calls the underlying provider's disable_monitoring on the stored search ID. Manual pauses are preserved across archive/restore cycles — restoring an archived identity does NOT re-enable monitoring that was manually paused beforehand. To bulk-disable monitoring across many identities, archive them instead. The archive cascade pauses monitoring with `reason='archived'` so a later restore re-enables it automatically.