Move our Stripe integration over to using Stripe Checkout so we can use Stripe's Chargeback Protection service to avoid chargebacks

### Epic

_No response_

### Description

**As a:**

Person who likes FlowForge to get paid for the services they provide.

**I want to:**

Update FlowForge's Stripe integration to use Stripe Checkout https://stripe.com/docs/payments/checkout

**So that:**

FlowForge can use Stripe's Chargeback Protection service - https://stripe.com/gb/radar/chargeback-protection

### Which customers would this be availble to

None

### Acceptance Criteria

_No response_

Start by reviewing the Stripe Checkout and Chargeback Protection links in the issue, then locate FlowFuse's existing Stripe integration. Define the affected customer flows and acceptance criteria before making changes; done means the integration uses Stripe Checkout and supports the intended chargeback-protection service.

Written by the indexing model from the issue text.