LITTER-MORTALITY: DeathsWithin8Weeks field (Frühsterblichkeit)

Litter.DeathsWithin8Weeks (int?, nullable). LitterDto + LitterInput extended.
Validation: negative → 400, > TotalBorn → 400 (skipped when TotalBorn null).
Migration AddLitterMortality (additive, nullable). 5 new tests. 194/194 green.
Contract: camelCase field deathsWithin8Weeks in LitterDto/LitterInput.

GerbilManager.Tests/LitterMortalityTests.cs

@@ -0,0 +1,88 @@
+using System.Net;
+using System.Net.Http.Json;
+using System.Text.Json;
+
+namespace GerbilManager.Tests;
+
+/// <summary>LITTER-MORTALITY: deathsWithin8Weeks field — round-trip, validation, null default.</summary>
+public class LitterMortalityTests : IClassFixture<ApiFactory>
+{
+    private readonly HttpClient _client;
+    public LitterMortalityTests(ApiFactory factory) => _client = factory.CreateClient();
+
+    private static int? GetInt(JsonElement el, string prop) =>
+        el.TryGetProperty(prop, out var v) && v.ValueKind == JsonValueKind.Number
+            ? v.GetInt32() : null;
+
+    private async Task<JsonElement> CreateAndGet(object body)
+    {
+        var post = await _client.PostAsJsonAsync("/litters", body);
+        post.EnsureSuccessStatusCode();
+        var idStr = JsonDocument.Parse(await post.Content.ReadAsStringAsync()).RootElement
+            .GetProperty("id").GetString()!;
+        return JsonDocument.Parse(await _client.GetStringAsync($"/litters/{idStr}")).RootElement;
+    }
+
+    [Fact]
+    public async Task DeathsWithin8Weeks_round_trips()
+    {
+        var el = await CreateAndGet(new
+        {
+            name = "Wurf Mortalitätstest",
+            date = "2026-05-01",
+            totalBorn = 6,
+            deathsWithin8Weeks = 2,
+        });
+        Assert.Equal(6, GetInt(el, "totalBorn"));
+        Assert.Equal(2, GetInt(el, "deathsWithin8Weeks"));
+    }
+
+    [Fact]
+    public async Task DeathsWithin8Weeks_defaults_null()
+    {
+        var el = await CreateAndGet(new { name = "Wurf Kein Mort", date = "2026-05-02" });
+        Assert.True(el.TryGetProperty("deathsWithin8Weeks", out var v));
+        Assert.Equal(JsonValueKind.Null, v.ValueKind);
+    }
+
+    [Fact]
+    public async Task DeathsWithin8Weeks_exceeding_TotalBorn_returns_400()
+    {
+        var resp = await _client.PostAsJsonAsync("/litters", new
+        {
+            name = "Ungültig",
+            date = "2026-05-03",
+            totalBorn = 4,
+            deathsWithin8Weeks = 5,
+        });
+        Assert.Equal(HttpStatusCode.BadRequest, resp.StatusCode);
+        var body = await resp.Content.ReadAsStringAsync();
+        Assert.Contains("TotalBorn", body);
+    }
+
+    [Fact]
+    public async Task Negative_DeathsWithin8Weeks_returns_400()
+    {
+        var resp = await _client.PostAsJsonAsync("/litters", new
+        {
+            name = "Negativ",
+            date = "2026-05-04",
+            deathsWithin8Weeks = -1,
+        });
+        Assert.Equal(HttpStatusCode.BadRequest, resp.StatusCode);
+        var body = await resp.Content.ReadAsStringAsync();
+        Assert.Contains("negativ", body);
+    }
+
+    [Fact]
+    public async Task DeathsWithin8Weeks_allowed_when_TotalBorn_null()
+    {
+        var el = await CreateAndGet(new
+        {
+            name = "Kein TotalBorn",
+            date = "2026-05-05",
+            deathsWithin8Weeks = 3,
+        });
+        Assert.Equal(3, GetInt(el, "deathsWithin8Weeks"));
+    }
+}