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/ApplicationContextModelSnapshot.cs

@@ -924,6 +924,9 @@ namespace GerbilManagerWebAPI.Migrations
                     b.Property<DateOnly>("Date")
                         .HasColumnType("date");
 
+                    b.Property<int?>("DeathsWithin8Weeks")
+                        .HasColumnType("integer");
+
                     b.Property<DateOnly?>("ExpectedGoHomeDate")
                         .HasColumnType("date");