Fix 4 post-review issues found in accounting module audit

- Drop orphan VendorCreditId1 column from VendorCreditApplications (was
  scaffolded by EF because WithMany() lacked inverse navigation name;
  fixed WithMany() → WithMany(vc => vc.Applications) in ApplicationDbContext)
- Wire EarlyPaymentDiscount fields through full data path: added
  EarlyPaymentDiscountPercent/Days to CreateInvoiceDto, hidden inputs to
  Invoice Create view, and JS to populate from customer AJAX response
- Add missing [HttpGet] attribute to TaxRatesController.Index
- Document GenerateNow architecture exception with XML rationale

Migration DropOrphanVendorCreditId1 applied. Build: 0 errors, 168 warnings.
Unit tests: 200/200 passing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Views/Invoices/Create.cshtml

@@ -59,6 +59,8 @@
     <input type="hidden" asp-for="PreparedById" />
     <input type="hidden" asp-for="JobId" />
     <input type="hidden" asp-for="CustomerId" id="hiddenCustomerId" />
+    <input type="hidden" asp-for="EarlyPaymentDiscountPercent" id="EarlyPaymentDiscountPercent" />
+    <input type="hidden" asp-for="EarlyPaymentDiscountDays" id="EarlyPaymentDiscountDays" />
 
     <div class="row g-4">
         <!-- LEFT: Main form -->
@@ -473,8 +475,10 @@
                     // Trigger due date recalculation (invoice-due-date.js listens to 'change')
                     termsSelect.dispatchEvent(new Event('change'));
                 }
-                // Show/hide early payment discount notice
+                // Show/hide early payment discount notice and persist values
                 const discountEl = document.getElementById('earlyPaymentDiscountNotice');
+                const discountPctField  = document.getElementById('EarlyPaymentDiscountPercent');
+                const discountDaysField = document.getElementById('EarlyPaymentDiscountDays');
                 if (discountEl) {
                     if (data.earlyPaymentDiscountPercent > 0) {
                         discountEl.textContent = `${data.earlyPaymentDiscountPercent}% discount if paid within ${data.earlyPaymentDiscountDays} days`;
@@ -483,6 +487,8 @@
                         discountEl.classList.add('d-none');
                     }
                 }
+                if (discountPctField)  discountPctField.value  = data.earlyPaymentDiscountPercent ?? 0;
+                if (discountDaysField) discountDaysField.value = data.earlyPaymentDiscountDays ?? 0;
             }).catch(() => {});
 
         // Fetch tax rate for the selected customer