HA: Enable HA feature to be billable

With #2156, the HA feature has been added in Preview mode without any billing integration.

Before we make the feature generally available (ie, not in preview) we need to integrate it into billing.

Copying notes over from 2156:

### Billing configuration

From previous discussions, the decision is for HA to be considered as an add-on item on the invoice, rather than use an alternative instance product.

In the current model, we have a `ProjectType` which includes the stripe price/product for that type.

The most straight-forward implementation will be to add HA price/product options in the `ProjectType` properties. This won't easily scale if we want different prices for different `TeamTiers`. So depending on how this item is scheduled we affect how it gets implemented.

Start by reviewing #2156 and the current ProjectType billing configuration, including how Stripe prices and products are represented. Decide how the HA add-on should account for TeamTiers, then trace the existing billing flow. Done means HA is integrated as a billable invoice add-on and is ready for general availability.

Written by the indexing model from the issue text.