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.Infrastructure/Data/ApplicationDbContext.cs

@@ -667,7 +667,7 @@ public class ApplicationDbContext : IdentityDbContext<ApplicationUser>, IDataPro
             .OnDelete(DeleteBehavior.NoAction);
         modelBuilder.Entity<VendorCreditApplication>()
             .HasOne(vca => vca.VendorCredit)
-            .WithMany()
+            .WithMany(vc => vc.Applications)
             .HasForeignKey(vca => vca.VendorCreditId)
             .OnDelete(DeleteBehavior.NoAction);