Updates an existing management contract, modifying terms, fees, or status.

This endpoint allows modification of the financial and contractual parameters
of the management agreement. Only the fields that need to be updated should be
included in the request.

Changes to fee structures will affect future calculations but will not retroactively
modify existing financial transactions.

Landlords (profile_contracts_attributes):
Each entry upserts a landlord on the contract:

• a profile already attached to the contract has its bank_account_id and share updated;
• a profile not yet attached is added to the contract. When the contract ends up with
  several landlords, the owners group is handled automatically server-side.

bank_account_id is optional (null is accepted for a landlord without a bank account).
share is only overwritten when provided (defaults to 1 for a newly added landlord).
Landlords absent from the payload are left untouched — this endpoint never detaches an owner.