Simulate Autopay for the tenant with the given external ID.

Requirements:

• The user must be approved (active membership) and enrolled in Autopay.
• The current mocked date must be the 1st of the month, or the last working day before it
  if the 1st falls on a weekend or bank holiday.

⚠️ Returns 400 if any of the above requirements are not met.

Set simulate_failure to true to simulate an Autopay failure.