Add BatchId to GiftCertificate for persistent bulk batch tracking

BatchId (Guid?) is stamped on every certificate in a bulk run so the batch
is permanently addressable. BulkResult is now a bookmarkable GET by batchId
rather than TempData, so users can return to re-download at any time.
BatchDownloadPdf is a GET link (no form POST needed). Index shows a Batch
badge on bulk certs that links directly back to the batch result page.

Migration: AddGiftCertificateBatchId

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

src/PowderCoating.Infrastructure/Migrations/20260515003030_AddGiftCertificateBatchId.cs

@@ -0,0 +1,71 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace PowderCoating.Infrastructure.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddGiftCertificateBatchId : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<Guid>(
+                name: "BatchId",
+                table: "GiftCertificates",
+                type: "uniqueidentifier",
+                nullable: true);
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 1,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 15, 0, 30, 26, 297, DateTimeKind.Utc).AddTicks(7656));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 2,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 15, 0, 30, 26, 297, DateTimeKind.Utc).AddTicks(7662));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 3,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 15, 0, 30, 26, 297, DateTimeKind.Utc).AddTicks(7664));
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "BatchId",
+                table: "GiftCertificates");
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 1,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 14, 20, 43, 43, 116, DateTimeKind.Utc).AddTicks(7475));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 2,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 14, 20, 43, 43, 116, DateTimeKind.Utc).AddTicks(7481));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 3,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 14, 20, 43, 43, 116, DateTimeKind.Utc).AddTicks(7482));
+        }
+    }
+}

src/PowderCoating.Infrastructure/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -3290,6 +3290,9 @@ namespace PowderCoating.Infrastructure.Migrations
 
                     SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
 
+                    b.Property<Guid?>("BatchId")
+                        .HasColumnType("uniqueidentifier");
+
                     b.Property<string>("CertificateCode")
                         .IsRequired()
                         .HasColumnType("nvarchar(450)");
@@ -6708,7 +6711,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 1,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 14, 20, 43, 43, 116, DateTimeKind.Utc).AddTicks(7475),
+                            CreatedAt = new DateTime(2026, 5, 15, 0, 30, 26, 273, DateTimeKind.Utc).AddTicks(2464),
                             Description = "Standard pricing for regular customers",
                             DiscountPercent = 0m,
                             IsActive = true,
@@ -6719,7 +6722,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 2,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 14, 20, 43, 43, 116, DateTimeKind.Utc).AddTicks(7481),
+                            CreatedAt = new DateTime(2026, 5, 15, 0, 30, 26, 273, DateTimeKind.Utc).AddTicks(2473),
                             Description = "5% discount for preferred customers",
                             DiscountPercent = 5m,
                             IsActive = true,
@@ -6730,7 +6733,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 3,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 14, 20, 43, 43, 116, DateTimeKind.Utc).AddTicks(7482),
+                            CreatedAt = new DateTime(2026, 5, 15, 0, 30, 26, 273, DateTimeKind.Utc).AddTicks(2474),
                             Description = "10% discount for premium customers",
                             DiscountPercent = 10m,
                             IsActive = true,