Fix/invoice numbers (#365)

* feat: make invoice_number nullable and assign on send

- Updated the invoices table to allow invoice_number to be nullable.
- Modified the logic to assign invoice numbers only when the invoice status transitions to 'sent'.
- Refactored related code to handle nullable invoice numbers, including UI components and API routes.
- Added tests to ensure correct behavior when handling invoices with null invoice numbers.
- Introduced a utility function to display invoice numbers, defaulting to '(Utkast)' for drafts.

* fix: update fiscal period handling to return names of open periods in error messages

* fix: enhance period creation logic to account for company-wide bookkeeping lock-through

* fix: remove unnecessary customer_type field from customer insertion query

* fix: scope invoice number count query to specific companies to avoid test interference

* feat: Implement atomic invoice number generation and ensure compliance with invoice numbering rules

- Introduced `ensureInvoiceNumber` function to assign invoice numbers atomically, handling concurrency and ensuring compliance with document types.
- Updated invoice-related components to utilize the new `invoiceNumberDisplay` utility for consistent invoice number formatting.
- Added checks to ensure that invoices in non-draft statuses have valid invoice numbers, preventing violations of legal requirements.
- Created tests for the new invoice number generation logic, ensuring correct behavior under various scenarios, including concurrent requests.
- Added a draft banner to PDF templates for invoices without assigned numbers, clarifying their status to users.
- Updated database migrations to support the new atomic invoice number generation logic and enforce constraints on invoice statuses.

components/invoices/PaymentBookingDialog.tsx

@@ -212,7 +212,7 @@ export default function PaymentBookingDialog({
     <Dialog open={open} onOpenChange={onOpenChange}>
       <DialogContent className="sm:max-w-[680px]">
         <DialogHeader>
-          <DialogTitle>Bokför betalning — {invoice.invoice_number}</DialogTitle>
+          <DialogTitle>Bokför betalning{invoice.invoice_number ? ` — ${invoice.invoice_number}` : ''}</DialogTitle>
           <DialogDescription>
             {formatCurrency(invoice.total, invoice.currency)}
             {invoice.currency !== 'SEK' && invoice.total_sek && (

components/invoices/SendInvoiceDialog.tsx

@@ -188,7 +188,7 @@ export default function SendInvoiceDialog({
       <DialogContent className="sm:max-w-[600px]">
         <DialogHeader>
           <DialogTitle>
-            {mode === 'email' ? 'Skicka faktura' : 'Markera som skickad'} — {invoice.invoice_number}
+            {mode === 'email' ? 'Skicka faktura' : 'Markera som skickad'}{invoice.invoice_number ? ` — ${invoice.invoice_number}` : ''}
           </DialogTitle>
           <DialogDescription>
             {formatCurrency(invoice.total, invoice.currency)}
@@ -215,7 +215,7 @@ export default function SendInvoiceDialog({
                 <JournalEntryReviewContent
                   periodName={periodName}
                   entryDate={invoice.invoice_date}
-                  description={`Försäljning faktura ${invoice.invoice_number}${invoice.customer.name ? `, ${invoice.customer.name}` : ''}`}
+                  description={`Försäljning faktura${invoice.invoice_number ? ` ${invoice.invoice_number}` : ''}${invoice.customer.name ? `, ${invoice.customer.name}` : ''}`}
                   lines={proposedLines}
                   totalDebit={totalDebit}
                   totalCredit={totalCredit}