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/Models/Litter.cs

@@ -13,6 +13,10 @@ namespace GerbilManagerWebAPI.Models
         /// <summary>Total born count (was "Strength").</summary>
         public int? TotalBorn { get; set; }
 
+        /// <summary>Frühsterblichkeit: Anzahl der in den ersten 8 Wochen verstorbenen Tiere.
+        /// Null = unbekannt (Altdaten). Must be ≤ TotalBorn if both are set.</summary>
+        public int? DeathsWithin8Weeks { get; set; }
+
         public Guid? FatherId { get; set; }
         public Gerbil? Father { get; set; }
         public Guid? MotherId { get; set; }