Stable product identity and developer slugs across environments

Creem product IDs differ between test and production, which makes billing catalogs harder to manage across environments.

It would be helpful if products had a developer-owned slug/key, for example pro, basic-team, or premium-individual.

This slug should be a stable part of the product identity, so apps can reference products predictably instead of relying only on environment-specific IDs.

This would reduce environment-specific mapping and make pricing, checkout, and entitlement logic easier to keep reliable.