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

components/import/SIEUploadStep.tsx

@@ -301,7 +301,7 @@ export default function SIEUploadStep({ onFileSelect, isLoading, error, errorTyp
             <p className="text-muted-foreground">Inställningar → Import/Export → Exportera SIE</p>
           </div>
           <div>
-            <p className="font-medium">Visma eEkonomi</p>
+            <p className="font-medium">Visma</p>
             <p className="text-muted-foreground">Rapporter → Övrigt → Exportera till SIE</p>
           </div>
           <div>