Support upgrade paths for one-time products

Creem currently supports upgrade/downgrade paths for subscriptions, but the same concept would be extremely valuable for one-time purchases such as Figma kits, templates, and software licenses.

Today, the workaround is to create separate “upgrade products” that only represent the price delta between two products.

This creates two major problems:

1. Product explosion
   A small catalog of 6 actual products can easily grow to 15+ products once all upgrade combinations are added.

2. Complex entitlement mapping
   The app can no longer rely on a simple 1:1 Creem product → internal plan mapping, because upgrade products need special handling, mapping tables, or an additional derived plan field.

Native one-time product upgrade paths would let Creem model these transitions directly, similar to subscription upgrades.

Example:

• Individual: A → B → C

• Team: A → B → C

• Cross-upgrades: Individual → Team

This would dramatically simplify catalog management, entitlement logic, and customer upgrade UX.