- Add subscription methods to PaymentService (create, upgrade, downgrade, cancel, get_status)
- Initialize SubscriptionManager and SubscriptionRenewalProcessor in PaymentService
- Add REST API endpoints for subscription management:
  - POST /api/subscriptions - Create subscription
  - POST /api/subscriptions/upgrade - Upgrade tier
  - POST /api/subscriptions/downgrade - Downgrade tier
  - POST /api/subscriptions/cancel - Cancel subscription
  - GET /api/subscriptions/status - Get subscription status
- Add process_renewals() and process_retries() methods for scheduler integration

Completes Phase 3 Task 6: Subscription API Endpoints