Implement Chargebee hosted checkout for subscription sign-up and handle post-checkout webhook confirmation

Steps

1. Generate a hosted page URL with POST /api/v2/hosted_pages/checkout_new_for_items; supply subscription_items[0][item_price_id] and customer[email]; the response contains the hosted_page url
2. Redirect the customer to the hosted page URL; Chargebee handles payment collection, SCA, and subscription creation
3. Configure the redirect_url on the hosted page request to specify where Chargebee sends the customer after successful checkout; the redirect includes a hosted_page_id query parameter
4. After redirect, verify the checkout outcome server-side with GET /api/v2/hosted_pages/HOSTED_PAGE_ID; confirm state=succeeded before provisioning access
5. Listen for subscription_created webhook to asynchronously provision entitlements; validate the webhook payload using the Chargebee-provided webhook password header
6. For a returning subscriber, use POST /api/v2/hosted_pages/checkout_existing_for_items with the existing subscription_id to allow plan upgrades/downgrades via hosted page