Closes the remaining #310 write paths: credit-note item copies (web, v1,
pending-operations) and arcim-migration supplier imports now normalize
vat_rate to the decimal-fraction unit before storage, and a NOT VALID
CHECK constraint guards every new supplier_invoice_items row. Customer
invoice items deliberately stay percent; legacy supplier rows are left
untouched so posted-entry reversals reuse the exact original values.
Fixes#310
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>