- Add downgrade_subscription() method to SubscriptionManager
  - Sets pending_tier_id for scheduled downgrade at period end
  - No immediate charge or refund
  - User keeps current tier until period end
  - Returns scheduled downgrade date
- Add cancel_subscription() method to SubscriptionManager
  - Sets cancel_at_period_end flag
  - User retains access until period end
  - No refund issued
  - Returns cancellation date
- Add comprehensive tests for both operations
  - Test downgrade scheduling and no-charge behavior
  - Test cancellation scheduling and no-refund behavior
  - Test error cases (no subscription, invalid tier)

All 12 subscription tests passing.