Add line-level VAT fields to a new invoices export

🔍 Problem Statement

As an EU finance user, I cannot prepare VAT reports from the invoices export, because it lacks VAT rate, VAT type and product code per line.

As a finance user, I want a new invoices export to show one row per invoice line with its VAT data.
As a finance user, I want credit note, refund and discount rows to reference the original invoice and its quarter, so that VAT returns correct the right period.
As a finance user, I want customer type and VAT ID on each row, so that the ZM can be derived from the same export.

Add a new export for this enhancement (extension of invoices).
Given an invoice for a bundle plan, when exported, then each product and postage charge is a separate row with its own net amount, VAT amount and rate.
Given a credit note, refund or discount, when exported, then each row includes its own document number, the original invoice number and date, and the original reporting quarter.
Given a selected quarter, when the export is filtered to EU consumer lines, then it matches the OSS Individual Transactions report.
Given an invoice, when requested via [clarify endpoint, e.g. GET /invoices/:id], then the API returns the same line-level fields.
This change will impact the API and export layers, specifically the invoices export and invoice line items.