Allow limiting affiliate commissions to a fixed timeframe (instead of lifetime)

Currently, affiliate commissions in Creem are always lifetime-based. Many SaaS businesses need the ability to limit commissions to a defined period, such as the first 3, 6, or 12 months after a customer signs up.

Please add an option to configure a commission timeframe per affiliate program (e.g. first X months or billing cycles). This would make affiliate payouts more predictable and align with common SaaS affiliate models.