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);
 

src/PowderCoating.Infrastructure/Migrations/20260510152530_DropOrphanVendorCreditId1.cs

@@ -0,0 +1,91 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace PowderCoating.Infrastructure.Migrations
+{
+    /// <inheritdoc />
+    public partial class DropOrphanVendorCreditId1 : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropForeignKey(
+                name: "FK_VendorCreditApplications_VendorCredits_VendorCreditId1",
+                table: "VendorCreditApplications");
+
+            migrationBuilder.DropIndex(
+                name: "IX_VendorCreditApplications_VendorCreditId1",
+                table: "VendorCreditApplications");
+
+            migrationBuilder.DropColumn(
+                name: "VendorCreditId1",
+                table: "VendorCreditApplications");
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 1,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 10, 15, 25, 26, 831, DateTimeKind.Utc).AddTicks(199));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 2,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 10, 15, 25, 26, 831, DateTimeKind.Utc).AddTicks(205));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 3,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 10, 15, 25, 26, 831, DateTimeKind.Utc).AddTicks(206));
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<int>(
+                name: "VendorCreditId1",
+                table: "VendorCreditApplications",
+                type: "int",
+                nullable: true);
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 1,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 10, 15, 1, 54, 769, DateTimeKind.Utc).AddTicks(6262));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 2,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 10, 15, 1, 54, 769, DateTimeKind.Utc).AddTicks(6270));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 3,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 10, 15, 1, 54, 769, DateTimeKind.Utc).AddTicks(6271));
+
+            migrationBuilder.CreateIndex(
+                name: "IX_VendorCreditApplications_VendorCreditId1",
+                table: "VendorCreditApplications",
+                column: "VendorCreditId1");
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_VendorCreditApplications_VendorCredits_VendorCreditId1",
+                table: "VendorCreditApplications",
+                column: "VendorCreditId1",
+                principalTable: "VendorCredits",
+                principalColumn: "Id");
+        }
+    }
+}

src/PowderCoating.Infrastructure/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -6293,7 +6293,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 1,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 10, 15, 1, 54, 769, DateTimeKind.Utc).AddTicks(6262),
+                            CreatedAt = new DateTime(2026, 5, 10, 15, 25, 26, 831, DateTimeKind.Utc).AddTicks(199),
                             Description = "Standard pricing for regular customers",
                             DiscountPercent = 0m,
                             IsActive = true,
@@ -6304,7 +6304,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 2,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 10, 15, 1, 54, 769, DateTimeKind.Utc).AddTicks(6270),
+                            CreatedAt = new DateTime(2026, 5, 10, 15, 25, 26, 831, DateTimeKind.Utc).AddTicks(205),
                             Description = "5% discount for preferred customers",
                             DiscountPercent = 5m,
                             IsActive = true,
@@ -6315,7 +6315,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 3,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 10, 15, 1, 54, 769, DateTimeKind.Utc).AddTicks(6271),
+                            CreatedAt = new DateTime(2026, 5, 10, 15, 25, 26, 831, DateTimeKind.Utc).AddTicks(206),
                             Description = "10% discount for premium customers",
                             DiscountPercent = 10m,
                             IsActive = true,
@@ -8171,17 +8171,12 @@ namespace PowderCoating.Infrastructure.Migrations
                     b.Property<int>("VendorCreditId")
                         .HasColumnType("int");
 
-                    b.Property<int?>("VendorCreditId1")
-                        .HasColumnType("int");
-
                     b.HasKey("Id");
 
                     b.HasIndex("BillId");
 
                     b.HasIndex("VendorCreditId");
 
-                    b.HasIndex("VendorCreditId1");
-
                     b.ToTable("VendorCreditApplications");
                 });
 
@@ -9883,15 +9878,11 @@ namespace PowderCoating.Infrastructure.Migrations
                         .IsRequired();
 
                     b.HasOne("PowderCoating.Core.Entities.VendorCredit", "VendorCredit")
-                        .WithMany()
+                        .WithMany("Applications")
                         .HasForeignKey("VendorCreditId")
                         .OnDelete(DeleteBehavior.NoAction)
                         .IsRequired();
 
-                    b.HasOne("PowderCoating.Core.Entities.VendorCredit", null)
-                        .WithMany("Applications")
-                        .HasForeignKey("VendorCreditId1");
-
                     b.Navigation("Bill");
 
                     b.Navigation("VendorCredit");