fix(vat): enforce decimal vat_rate on supplier invoice items and normalize MCP percent extraction (#1049)

Supplier invoice items store vat_rate as a decimal fraction (0.25) while
customer invoices use integer percent (25). The shared Zod schema accepted
0-100, so a percent-shaped vat_rate silently booked 2500 % VAT via
line_total * vat_rate, and the MCP inbox-conversion path staged the AI
extraction's percent-integer vatRate straight into the decimal column with
per-line vat_amount 0. Part of #310.

- CreateSupplierInvoiceItemSchema.vat_rate is now a literal union of the
  statutory decimal set (0, 0.06, 0.12, 0.25) with a unit-hint error,
  covering the cookie route, the invoice-inbox convert route, and /api/v1
  (whose runtime ALLOWED_SV_VAT_RATES guard stays as defense in depth).
- New shared normalizeVatRateToDecimal() in lib/vat: percent-shaped values
  (25, 12, 6) divide by 100, results snap to the legal Swedish set, and
  anything else (foreign 19/20, non-finite) maps to 0.
- gnubok_create_supplier_invoice_from_inbox normalizes vatRate at the
  extraction boundary and derives per-line vat_amount when the extraction
  carries none, so the staged header vat_amount is honest.
- The pending-operation executor normalizes staged vat_rate on insert, so
  rows staged before this fix cannot book percent-scaled VAT.

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

lib/api/__tests__/schemas.test.ts

@@ -831,6 +831,27 @@ describe('CreateSupplierInvoiceItemSchema', () => {
     }
   })
 
+  it('rejects percent-shaped or non-statutory vat_rate (decimal convention, issue #310)', () => {
+    // 25/12/6 are the percent-integer shape (books 2500 % VAT if accepted),
+    // 0.19 is a foreign decimal rate, 100 is the old max() boundary.
+    for (const rate of [25, 12, 6, 0.19, 100]) {
+      const result = CreateSupplierInvoiceItemSchema.safeParse(
+        validSupplierInvoiceItem({ vat_rate: rate })
+      )
+      expect(result.success).toBe(false)
+    }
+  })
+
+  it('rejects percent-shaped vat_rate with a unit hint in the message', () => {
+    const result = CreateSupplierInvoiceItemSchema.safeParse(
+      validSupplierInvoiceItem({ vat_rate: 25 })
+    )
+    expect(result.success).toBe(false)
+    if (!result.success) {
+      expect(result.error.issues[0].message).toMatch(/decimal fraction/)
+    }
+  })
+
   it('accepts vat_amount up to line_total * vat_rate', () => {
     const result = CreateSupplierInvoiceItemSchema.safeParse(
       validSupplierInvoiceItem({ amount: 5000, vat_rate: 0.25, vat_amount: 1250 })