Feat/voucher docs (#664)

* feat: implement inbox document picker and linking functionality

* feat: implement self-billing invoice functionality

- Added support for registering self-billed invoices received from customers.
- Updated the invoice schema to include fields for self-billing metadata such as `is_self_billed`, `external_invoice_number`, `self_billing_agreement_ref`, and `received_date`.
- Created API route for handling self-billed invoice submissions, including validation and error handling.
- Implemented database migrations to add necessary columns and constraints for self-billing invoices.
- Developed tests to ensure correct behavior of self-billing invoice creation and validation rules.
- Updated Swedish localization files to include new terms related to self-billing.

* feat: enforce SIE import requirement for non-Fortnox providers in migration process

* feat: streamline invoice processing and enhance error logging across APIs

lib/reports/ar-ledger.ts

@@ -129,7 +129,9 @@ export async function generateARLedger(
     // Add invoice detail (always — even if unconvertible, so it's visible)
     entry.invoices.push({
       invoice_id: inv.id,
-      invoice_number: inv.invoice_number || '',
+      // Self-billing invoices we received have no own number — show the
+      // counterparty's external number instead.
+      invoice_number: inv.invoice_number || inv.external_invoice_number || '',
       invoice_date: inv.invoice_date || '',
       due_date: inv.due_date,
       total,