feat(serial-code): add SerialCode + SerialCodeReward + ViewerSerialCodeRedemption entities

Three new EF entities for /campaign/regist_serial_code: SerialCodeEntry (code, message,
window, enabled flag), SerialCodeRewardEntry (FK child, per-slot reward), and
ViewerSerialCodeRedemption (composite-PK redemption record). Registered in SVSimDbContext
with unique index on Code and cascade FK constraints. 3/3 persistence tests pass.

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

SVSim.UnitTests/Persistence/SerialCodePersistenceTests.cs

@@ -0,0 +1,92 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection;
+using SVSim.Database;
+using SVSim.Database.Models;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Persistence;
+
+public class SerialCodePersistenceTests
+{
+    [Test]
+    public async Task SerialCode_round_trips_with_rewards()
+    {
+        using var factory = new SVSimTestFactory();
+        using (var seedScope = factory.Services.CreateScope())
+        {
+            var ctx = seedScope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+            ctx.SerialCodes.Add(new SerialCodeEntry
+            {
+                Code = "ABCD-1234",
+                Message = "Test code",
+                IsEnabled = true,
+                Rewards =
+                {
+                    new SerialCodeRewardEntry { Slot = 0, RewardType = 1, RewardDetailId = 0, RewardCount = 100 },
+                    new SerialCodeRewardEntry { Slot = 1, RewardType = 9, RewardDetailId = 0, RewardCount = 500 },
+                },
+            });
+            await ctx.SaveChangesAsync();
+        }
+
+        using var verifyScope = factory.Services.CreateScope();
+        var ctx2 = verifyScope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+        var code = await ctx2.SerialCodes
+            .Include(c => c.Rewards.OrderBy(r => r.Slot))
+            .AsNoTracking()
+            .FirstAsync(c => c.Code == "ABCD-1234");
+
+        Assert.That(code.Message, Is.EqualTo("Test code"));
+        Assert.That(code.IsEnabled, Is.True);
+        Assert.That(code.Rewards, Has.Count.EqualTo(2));
+        Assert.That(code.Rewards[0].RewardCount, Is.EqualTo(100));
+        Assert.That(code.Rewards[1].RewardCount, Is.EqualTo(500));
+    }
+
+    [Test]
+    public async Task Unique_constraint_on_Code_rejects_duplicates()
+    {
+        using var factory = new SVSimTestFactory();
+        using var scope = factory.Services.CreateScope();
+        var ctx = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+
+        ctx.SerialCodes.Add(new SerialCodeEntry { Code = "DUP", Message = "first", IsEnabled = true });
+        await ctx.SaveChangesAsync();
+
+        ctx.SerialCodes.Add(new SerialCodeEntry { Code = "DUP", Message = "second", IsEnabled = true });
+        Assert.That(async () => await ctx.SaveChangesAsync(), Throws.Exception);
+    }
+
+    [Test]
+    public async Task Composite_PK_on_redemption_rejects_double_redeem()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+
+        int codeId;
+        using (var seedScope = factory.Services.CreateScope())
+        {
+            var ctx = seedScope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+            var code = new SerialCodeEntry { Code = "ONCE", Message = "single use", IsEnabled = true };
+            ctx.SerialCodes.Add(code);
+            await ctx.SaveChangesAsync();
+            codeId = code.Id;
+
+            ctx.ViewerSerialCodeRedemptions.Add(new ViewerSerialCodeRedemption
+            {
+                ViewerId = viewerId, SerialCodeId = codeId, RedeemedAt = DateTime.UtcNow,
+            });
+            await ctx.SaveChangesAsync();
+        }
+
+        // Second redemption attempt in a fresh scope so the change tracker doesn't intercept
+        // before the DB constraint fires.
+        using var dupeScope = factory.Services.CreateScope();
+        var ctx2 = dupeScope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+        ctx2.ViewerSerialCodeRedemptions.Add(new ViewerSerialCodeRedemption
+        {
+            ViewerId = viewerId, SerialCodeId = codeId, RedeemedAt = DateTime.UtcNow,
+        });
+        Assert.That(async () => await ctx2.SaveChangesAsync(), Throws.Exception);
+    }
+}