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

types/index.ts

@@ -762,6 +762,21 @@ export interface Invoice {
   // Conversion tracking (proforma -> invoice)
   converted_from_id: string | null
 
+  // Self-billing received (mottagen självfaktura, ML 17 kap 15§). When
+  // `is_self_billed` is true the customer issued the invoice on our behalf;
+  // for us it is a sale. The counterparty's number lives in
+  // `external_invoice_number` and our own `invoice_number` stays null so we
+  // never consume our löpnummerserie (BFL 5 kap 6§).
+  is_self_billed?: boolean
+  external_invoice_number?: string | null
+  self_billing_agreement_ref?: string | null
+  received_date?: string | null
+
+  // Verifikation produced when the invoice was booked (registration entry).
+  // Lets the payment flow detect an already-booked sale and clear 1510 rather
+  // than re-recognising revenue.
+  journal_entry_id?: string | null
+
   // Payment tracking
   paid_at: string | null
   paid_amount: number | null