Membership free-trial discounts now apply correctly on shops with many plans

Membership plans in Subi use a Shopify discount function to apply free-trial pricing. That function reads plan configuration from a metafield. When a shop's membership configuration metafield exceeded Shopify's 10,000-byte limit for function input data, Shopify omitted the metafield silently -- the function received nothing, assumed no plans were configured, and stopped applying free-trial discounts across all plans on the shop at once, with no error shown anywhere.

The fix trims the configuration payload to include only plans that have a free-trial period configured (free_trial_days > 0). Plans without a free trial can never produce a discount operation and were taking up space in the metafield unnecessarily. This keeps the payload well within the limit on shops with many membership plans.

If you run a membership program with a large number of plans and noticed free trials stop applying, this update restores the correct behavior.