Add-ons for seat-based billing

Summary

Support add-ons on seat-based (per-seat) subscriptions.

Current behavior

Add-ons can be attached to standard recurring plans, but not to seat-based billing subscriptions.

Request

Allow merchants to attach add-ons to seat-based / per-seat subscriptions — so a customer on a per-seat plan can also purchase add-ons alongside their seats.

Why it matters

• Seat-based products often need optional extras (premium features, extra capacity, support tiers) sold as add-ons.
• Without this, merchants on seat-based billing can't monetize add-ons the way standard-plan merchants can.

Notes

Relates to the existing gap where add-ons are limited to standard recurring plans (see also: add-ons not supported on usage-based billing). This request covers the seat-based case specifically.

Captured by the team from roadmap planning.