adds a /pay/{token} redirect endpoint that generates a short URL
pointing to a stripe checkout sessions in setup mode. The TUI shows a QR
code and URL on the payment page so users can add a card via the browser
instead of entering raw card data over SSH.
the checkout.session.completed webhook saves the card to the database.
the TUI polls GET /api/v1/cards every second and auto-proceeds to
confirmation once the new card is detected.
also adds a dev.sh startup script that runs all four required processes.
(SSH server, API server, Stripe CLI listener, ngrok) in a single
terminal
