- Remove User.stripe_id column (no longer used with Stripe Checkout)
- Show billing link to all authenticated users in phone menu
- Remove old Stripe customer cleanup from tests
- Add test_stripe.py with unit tests for checkout module and Payment model
- Add functional tests for billing views (pay-what-you-can, checkout, success)
- Fix mock imports for Python 3 compatibility (unittest.mock)
- Remove unused stripe.j2 imports from templates