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.

GerbilManagerWebAPI/Migrations/20260607013052_AddLitterMortality.cs

@@ -0,0 +1,28 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace GerbilManagerWebAPI.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddLitterMortality : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<int>(
+                name: "DeathsWithin8Weeks",
+                table: "Litters",
+                type: "integer",
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "DeathsWithin8Weeks",
+                table: "Litters");
+        }
+    }
+}