Customers: Soft deletion does not free up the email

[birkjernstrom]

New issue to sync with Linear from #4816

Description

We soft delete customers, but the unique index on organization_id and email prevents the email from being freed up for a new customer.

Expected Behavior

We could probably solve this by modifying the index to include the deleted_at timestamp. Causing a unique violation in case of creating a customer with the same email as an existing – non-deleted – customer, but not in case others have a unique deleted_at timestamp.