Subscription and recurring billing logic

[Ogstevyn]

Subscription Engine — Recurring Billing for Merchants

Summary

Implements a Stripe-like recurring billing system on Soroban using a pull model. Merchants can create billing plans, customers can subscribe, and an authorized caller (merchant or bot) triggers charges once the billing interval has elapsed.

New Files

• contracts/shade/src/components/subscription.rs — full subscription engine implementation

Modified Files

• types.rs — added SubscriptionPlan, Subscription, SubscriptionStatus structs and SubscriptionPlan(u64), Subscription(u64), PlanCount, SubscriptionCount DataKey variants
• events.rs — fixed existing conflicts/incomplete sections; added SubscriptionPlanCreated, Subscribed, SubscriptionCharged, SubscriptionCancelled events
• interface.rs — added 6 new methods to ShadeTrait: create_subscription_plan, get_subscription_plan, subscribe, get_subscription, charge_subscription, cancel_subscription
• errors.rs — added 8 new error variants: NotFound, InvalidAmount, InvalidInterval, PlanNotActive, SubscriptionNotActive, IntervalNotElapsed, MerchantNotFound, MerchantAccountNotFound
• shade.rs — wired subscription functions into the contract impl block
• components/mod.rs — registered new subscription module

How It Works

Function	Who Calls It	What It Does
create_subscription_plan	Merchant	Creates a billing plan with token, amount, and interval
subscribe	Customer	Subscribes to a plan; customer must pre-approve token allowance
charge_subscription	Anyone (merchant/bot)	Pulls funds via transfer_from if interval has elapsed
cancel_subscription	Customer or Merchant	Cancels the recurring billing

Fee Model

Mirrors the existing invoice fee logic — fee is read in basis points from FeeInBasisPoints(token), deducted from the charge amount, and sent to the Shade contract. The net amount goes to the merchant's account contract.

Closes

closes #87