Changed the billing provider in the `.env.development` file from Stripe to Lemon Squeezy. This requires adaptations at many levels: at the web app to load Lemon Squeezy's script in the checkout process, at the billing gateway to handle Lemon Squeezy calls, and in the database to reflect the current billing provider. The checkout process is now done using Lemon Squeezy Sessions and its billing strategy was adjusted accordingly. Billing-related components and services were also updated.
Added implementation for various billing services with Lemon Squeezy. This includes processing of webhooks for order and subscription handling, verification of webhook signatures, and validating subscription statuses and order states. Additionally, types for order and subscription webhooks have been created.
