feat(customers): let users set a customer number shown on the invoice (#957)

Implements #914 (kundnummer on customers, printed on the invoice PDF).

- Migration: nullable text column customers.customer_number, no unique
  constraint in v1 so existing rows and imports keep working.
- API: CreateCustomerSchema/UpdateCustomerSchema accept an optional
  customer_number (trimmed, max 32 chars, nullable-then-optional so the
  OpenAPI registry sees it as not required); create/update routes
  persist it and normalize empty string to null so it can be cleared.
- v1 public API: customers create/detail/update round-trip the field
  (insert and update field lists, response projections, response
  schemas), and the invoices :send route fetches customer_number in its
  explicit customer join so the emailed PDF matches the downloaded one
  (the pdf route already selects customers(*)).
- UI: optional Kundnummer field in CustomerForm (next-intl keys in both
  sv and en), wired into the edit dialog's initialData; read-only
  Kundnummer row on the customer detail page's business-details card.
- Invoice PDF: renders "Kundnr:" / "Customer no.:" in the customer box
  when set; the PDF reads the live customers join, so no snapshot
  column is needed.
- Tests: route tests cover 400 validation, trimming, clearing with
  null/empty, and omit-leaves-untouched on POST and PATCH; v1 tests
  cover the create/update round-trip (insert/update payload + response
  projection) and the :send customer-join projection.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/api/schemas.ts

@@ -601,6 +601,14 @@ export const MarkInvoicePaidSchema = z.object({
 export const CreateCustomerSchema = z.object({
   name: z.string().min(1, 'Customer name is required'),
   customer_type: CustomerTypeSchema,
+  // Kundnummer shown on invoices. Free text, not unique in v1. Empty string
+  // and null both clear the value (routes normalize '' to null).
+  customer_number: z
+    .string()
+    .trim()
+    .max(32, 'Customer number must be 32 characters or fewer')
+    .nullable()
+    .optional(),
   email: z.string().email('Invalid email address').optional(),
   phone: z.string().optional(),
   address_line1: z.string().optional(),

lib/invoices/pdf-template.tsx

@@ -42,6 +42,7 @@ const LABELS = {
     yourReference: 'Er referens:',
     ourReference: 'Vår referens:',
     // Customer box
+    custNo: 'Kundnr:',
     orgNo: 'Org.nr:',
     vat: 'VAT:',
     // Table columns
@@ -111,6 +112,7 @@ const LABELS = {
     deliveryDate: 'Delivery date:',
     yourReference: 'Your reference:',
     ourReference: 'Our reference:',
+    custNo: 'Customer no.:',
     orgNo: 'Reg. no.:',
     vat: 'VAT:',
     colDescription: 'Description',
@@ -799,6 +801,12 @@ export function InvoicePDF({ invoice, customer, items, company, originalInvoiceN
               {customer.country && customer.country !== 'SE' && (
                 <Text>{customer.country}</Text>
               )}
+              {/* Seller-assigned kundnummer: no per-customer-type guard needed,
+                  it identifies the customer in the seller's own register and
+                  carries no personal data of its own. */}
+              {customer.customer_number && (
+                <Text style={{ marginTop: 6 }}>{L.custNo} {customer.customer_number}</Text>
+              )}
               {/* Suppress the identifier row for private customers: their
                   personnummer is not required on a B2C invoice (ML 17 kap 24§
                   asks for name + address only) and printing it is a GDPR