Set the monthly price of a subscription in a region (no price source exists)

Requires region. Body: {"monthly_price": 9.99, "currency": "USD", "source": "https://...", "source_date": "2026-10-09"}. source (up to 500 characters) and source_date (yyyy-MM-dd, not in the future) are optional and replace the stored values: omitting them clears the previous source.