Support hybrid pricing (base subscription + unit pricing)

It would be great if Creem supported hybrid pricing, where a subscription can have both a fixed recurring base price and a per-unit charge.

Example:

- $29/month base subscription
- + $5 per seat (or +$0.10 per API call)

Currently, it seems you can choose either a flat subscription or unit-based pricing, but not combine both. This is a common SaaS pricing model and would make Creem usable for many additional billing scenarios.