- 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