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.

types/index.ts

@@ -591,8 +591,8 @@ export interface Invoice {
   company_id: string
   customer_id: string
 
-  // Invoice number (auto-generated)
-  invoice_number: string
+  // Invoice number (auto-generated at first send; null while draft)
+  invoice_number: string | null
 
   // Dates
   invoice_date: string