Rotate the monitor webhook signing secret

Generates a new signing secret and returns it in this response. Store it securely. A retry with the original Idempotency-Key recovers the same secret for up to 24 hours while it remains the current secret or the valid previous secret. An expired or displaced secret returns 409 idempotency_result_expired without rotating again. Reads do not expose the secret. Zero-downtime overlap: the previous secret remains valid until previous_secret_expires_at (24 hours); during the window every delivery carries an additional X-Arcmira-Signature-Previous header computed with the old secret over the same {timestamp}.{payload} string, so you can verify with either secret while you roll. After the window the old secret is dropped and the extra header disappears. Rotating again during the window replaces the previous secret and resets the window. Requires a configured webhook (webhook_url set); otherwise 409 with code webhook_not_configured. Auto-disable interplay: rotation resets webhook_failures but never re-enables a webhook that was auto-disabled after repeated failures; to resume delivery, also PATCH the monitor with notify_webhook: true. Requires the monitors:write scope.