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.Application/DTOs/Invoice/InvoiceDtos.cs

@@ -82,6 +82,10 @@ public class CreateInvoiceDto
     public string? InternalNotes { get; set; }
     public string? Terms { get; set; }
     public string? CustomerPO { get; set; }
+    /// <summary>Early-payment discount percentage parsed from the customer's payment terms (e.g., 2.0 for "2/10 Net 30"). Informational — does not auto-apply.</summary>
+    public decimal EarlyPaymentDiscountPercent { get; set; }
+    /// <summary>Number of days within which the early-payment discount applies (e.g., 10 for "2/10 Net 30").</summary>
+    public int EarlyPaymentDiscountDays { get; set; }
     public List<CreateInvoiceItemDto> InvoiceItems { get; set; } = new();
 }