Advanced Filtering

New filter parameters for list endpoints:

Contracts (GET /contracts):

• status - Filter by contract status
• customer_id - Filter by customer
• start_date.gt, start_date.gte, start_date.lt, start_date.lte - Filter by start date
• end_date.gt, end_date.gte, end_date.lt, end_date.lte - Filter by end date

Products (GET /products):

• currency - Filter by price currency

Customers (GET /customers):

• external_reference - Filter by external reference