Add Stripe backend for listeners donation

[gammaw]

Corresponding frontend issue: lahmacunradio/frontend#305

Metadata requirements:

• Store campaign data (e.g., "Lahmacun radio – single payment (public donation page)")

Hint for adding metadata of one-time payments: https://stripe.com/docs/api/checkout/sessions/create#create_checkout_session-payment_intent_data-metadata

Hint for customising checkout sessions: https://stripe.com/docs/billing/subscriptions/build-subscriptions