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>

tests/helpers.ts

@@ -400,6 +400,7 @@ export function makeCustomer(overrides: Partial<Customer> = {}): Customer {
     company_id: 'company-1',
     name: 'Test AB',
     customer_type: 'swedish_business',
+    customer_number: null,
     email: 'kontakt@test.se',
     phone: null,
     address_line1: 'Storgatan 1',