fix(tax): derive applied_to_organization from billing entity

[toommz]

Context

GET /taxes (and the GraphQL Tax type) returned applied_to_organization: false for a tax applied to the organization's default billing entity, because it reads the deprecated taxes.applied_to_organization column directly. That column stays in sync only when taxes are managed through the tax endpoints, applying a tax via the billing-entity paths (PUT /billing_entities/:code, GraphQL billingEntities/applyTaxes) only changes the billing_entities_taxes join.

What changed

• applied_to_organization is now derived from the billing_entities_taxes join on the default billing entity, in both the REST serializer and the GraphQL type.
• New applied_to_billing_entities_codes array field exposes which billing entities a tax is applied to.
• The organization payload's tax list is selected the same way, so it agrees with the flag.