feat: multiple payment methods / default payments

[mileszim]

No description provided.

[what-the-diff]

• Added a new column to the users table called default_payment_method_id
• Changed the User model so that it has an association with payment methods, and added a method for getting this value
• Updated schema file to reflect changes in database structure (added foreign key)
• Updated factory bot user definition to include default payment method id as well