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/Dtos/ApiDtos.cs

@@ -36,6 +36,7 @@ namespace GerbilManagerWebAPI.Dtos
         string Name,
         DateOnly Date,
         int? TotalBorn,
+        int? DeathsWithin8Weeks,
         Guid? FatherId,
         Guid? MotherId,
         DateOnly? ExpectedGoHomeDate,
@@ -85,6 +86,7 @@ namespace GerbilManagerWebAPI.Dtos
         string Name,
         DateOnly Date,
         int? TotalBorn,
+        int? DeathsWithin8Weeks,
         Guid? FatherId,
         Guid? MotherId,
         DateOnly? ExpectedGoHomeDate,