- 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

Implements automatic subscription renewal processing with:
- RenewalProcessor class that finds and processes due subscriptions
- Extends billing periods by 30 days (monthly) or 365 days (yearly)
- Applies pending tier changes (downgrades) at renewal
- Handles subscription cancellations (cancel_at_period_end flag)
- Supports both fiat (card/PayPal) and crypto wallet payments
- Graceful handling of payment failures
- Comprehensive test coverage (8 tests, all passing)

Files:
- aisbf/payments/subscription/renewal.py: Core renewal processor
- tests/payments/test_renewal.py: Complete test suite
- aisbf/payments/subscription/__init__.py: Export RenewalProcessor

- 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.

- Add upgrade_subscription() method to SubscriptionManager
- Calculate prorated charge based on unused time in billing period
- Formula: new_price - (old_price × unused_fraction)
- Update subscription tier immediately while keeping same period end date
- Charge prorated amount via existing payment method
- Add comprehensive tests for proration calculations
- Test edge cases: no subscription, invalid tier, accurate proration math

- Create SubscriptionManager class with create_subscription method
- Validate tier and payment method before creating subscription
- Calculate amount based on billing cycle (monthly/yearly)
- Check for existing active subscription
- Calculate period dates with immediate start
- Charge payment via fiat (card/PayPal) or crypto
- Create subscription record and update user tier
- Add comprehensive test coverage

- Add fiat handler initialization (StripeHandler, PayPalHandler)
- Add add_stripe_payment_method() method
- Add initiate_paypal_billing_agreement() method
- Add complete_paypal_billing_agreement() method
- Add get_payment_methods() method to retrieve all user payment methods
- Add delete_payment_method() with validation to prevent deletion of methods used by active subscriptions

- Create StripePaymentHandler class with async operations
- Implement add_payment_method() with .00 authorization hold verification
- Authorization hold is immediately cancelled to release funds
- Get or create Stripe customer with metadata
- Store payment method in database
- Handle webhooks with signature verification
- Add stripe_customer_id column to users table
- Add comprehensive tests for Stripe integration