Expose pending units for proration-based subscription updates

When using seat-based subscriptions with proration-charge, subscription unit updates are only reflected after the charge successfully settles. Until then, API responses (including POST /v1/subscriptions/<id>) return stale unit counts, and the webhook becomes the only source of truth.

It would be very useful to expose the pending / requested unit count via the API (e.g. a pending_units field), so merchants can distinguish between active and requested capacity without relying solely on webhooks or duplicating state internally.

This would greatly simplify integrations for infrastructure or usage-based billing setups.