Backend: implement payment services

[nekopudding]

• create a customer on first session with user_id as metadata, store customer_id also in accountsDB
• create a subscription and attach the customer to it, set to default_incomplete and on checkout success activate the subscription
• once backend is uploaded, make the webhook redirect to the backend webhook url to listen for stripe events
• create a session attaching the customer and subscription and redirect the user to it when user wants to subscribe
• create a customer portal session when requested for the user to manage their subscription, attach the customer to it

Deliverables:

• code on github