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/Controllers/RecurringTemplatesController.cs

@@ -174,6 +174,15 @@ public class RecurringTemplatesController : Controller
     /// Forces immediate generation of the next occurrence and advances NextFireDate,
     /// regardless of the scheduled date. Useful for testing a new template or catching up
     /// after a configuration change.
+    /// <para>
+    /// INTENTIONAL EXCEPTION to the no-DbContext-in-controllers rule: this action must
+    /// execute the same multi-step entity creation that <see cref="RecurringTransactionService"/>
+    /// does in a background scope. Creating an inner DI scope here avoids duplicating that
+    /// service's interface (which would require exposing a public synchronous Fire method on a
+    /// singleton BackgroundService, which is unsafe). The scope is disposed after the action
+    /// completes, so no DbContext leak occurs. This pattern mirrors how BackgroundService
+    /// itself resolves scoped services.
+    /// </para>
     /// </summary>
     [HttpPost]
     [ValidateAntiForgeryToken]
@@ -184,6 +193,8 @@ public class RecurringTemplatesController : Controller
 
         try
         {
+            // Intentional: create a fresh DI scope so the inner DbContext is isolated from the
+            // request's IUnitOfWork context. See summary above for rationale.
             using var scope = HttpContext.RequestServices.CreateScope();
             var db = scope.ServiceProvider.GetRequiredService<PowderCoating.Infrastructure.Data.ApplicationDbContext>();
 

src/PowderCoating.Web/Controllers/TaxRatesController.cs

@@ -29,6 +29,7 @@ public class TaxRatesController : Controller
     }
 
     /// <summary>Lists all tax rates for the current company.</summary>
+    [HttpGet]
     public async Task<IActionResult> Index()
     {
         var rates = await _unitOfWork.TaxRates.GetAllAsync();