#201 feat(paykit): add subscription quantity support

Summary

• Add optional subscription quantity support to subscribe input and provider calls
• Pass quantity through Stripe checkout, create, update, and scheduled changes
• Sync subscription item quantity from provider/webhooks into the existing local subscription quantity column
• Add a changeset for paykitjs

Out of scope

• Checkout command idempotency keys
• Resuming open Checkout Sessions
• Automatic tax, Tax ID, or billing address collection
• App-specific organization or entitlement logic

Test Plan

• pnpm test:unit
• pnpm --filter paykitjs typecheck
• pnpm lint
• pnpm --filter paykitjs build

Summary by CodeRabbit

• New Features

  • Subscription flows now support item quantity across checkout, subscription creation, updates, and scheduled changes.
  • Quantity is accepted in subscribe requests, validated as a positive integer, and carried through to provider and local records.

• Bug Fixes

  • Quantity is now normalized and preserved consistently, including during billing sync and scheduled activations.

• Tests

  • Added unit and end-to-end Stripe provider coverage to confirm quantity is forwarded and reflected correctly (including schedule phase changes).