feat: implement Subscription and SubscriptionPlan contract entry points

[Pvsaint]

Problem

lib.rs defines Subscription, SubscriptionPlan, and SubscriptionStatus as #[contracttype] structs, and EVENTS.md documents a SUBSCRIPTION/CREATED event — but there are zero callable entry points for subscriptions anywhere in the codebase. This is dead code that will trigger Clippy warnings and confuse integrators.

Required Entry Points

• create_plan(merchant, plan_id, name, description, amount, currency, interval_secs) — merchant creates a billing plan
• subscribe(payer, subscription_id, plan_id) — payer subscribes to a plan
• process_subscription(operator, subscription_id) — operator triggers a recurring charge
• cancel_subscription(payer_or_merchant, subscription_id) — cancel an active subscription
• get_subscription(subscription_id) → Subscription
• get_plan(plan_id) → SubscriptionPlan

Acceptance Criteria

• All six entry points are implemented on a contract (PaymentProcessor or dedicated SubscriptionManager)
• process_subscription enforces next_payment_at — rejects if called before due date
• max_payments is respected; subscription auto-cancels when limit is reached
• SUBSCRIPTION/CREATED, SUBSCRIPTION/CHARGED, SUBSCRIPTION/CANCELLED events are emitted
• EVENTS.md and CHANGELOG.md are updated
• Unit tests cover: subscribe, charge on time, reject early charge, cancel, max_payments limit
• cargo test --all-features passes

Branch & Commit Examples

branch: feat/subscription-manager
commit: feat(subscriptions): implement create_plan, subscribe, process_subscription, cancel_subscription