Add approved invoice status with lock after customer acceptance.

Co-authored-by: Cursor <cursoragent@cursor.com>

database/migrations/041_invoice_status_approved.sql

@@ -0,0 +1,3 @@
+-- Add 'approved' to invoice_status (customer accepted the invoice; content becomes immutable).
+
+ALTER TYPE invoice_status ADD VALUE IF NOT EXISTS 'approved';