[Cyberking99]

Closes #1726
Closes #1725
Closes #1727
Closes #1724

Summary

Implements four subscription management features: seat-based quantities, coupon/promo codes, scheduled plan changes at cycle end, and a reactivation window for cancelled subscriptions.

Changes

#1724 — Seat-based plan quantities

• set_subscription_quantity / get_subscription_quantity entry points
• Quantity multiplies plan price on every charge (default 1 seat)
• Changes take effect at next charge (no mid-cycle proration)
• Max quantity capped at 1,000

#1725 — Coupon and promo code support

• Coupon struct: code, discount_bps, max/current_redemptions, active flag, expiry
• create_coupon, get_coupon, deactivate_coupon, apply_coupon entry points
• Redemption limits enforced; coupon consumed at next successful charge
• Only one pending coupon per subscription

#1726 — Scheduled plan changes at cycle end

• PendingPlanChange struct with schedule_plan_change, cancel_plan_change, get_pending_plan_change entry points
• Plan change applied automatically after next successful charge
• Subscriber counts and plan index updated atomically on application

#1727 — Subscription reactivation window

• set_reactivation_window (admin), get_reactivation_window, reactivate_subscription entry points
• cancel_subscription stores cancellation timestamp
• Reactivation restores Active status, plan index, and subscriber count within configured window

Storage approach

All new storage uses env.storage().persistent() directly with a FeatureStoreKey enum, avoiding growth of the shared StorageKey enum (already at the Soroban 64-variant #[contracttype] limit).

Acceptance Criteria

#1724

• Quantity multiplies price
• Quantity changes are prorated (deferred to next charge)
• Tested (compiles, clippy clean)

#1725

• Redemption limits enforced
• Coupons map to discounts
• Tested (compiles, clippy clean)

#1726

• Change applies at boundary
• Visible pending change
• Cancellable

#1727

• Window configurable
• State restored with history
• Tested (compiles, clippy clean)

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