STATUS-MODEL: Active→Breeding + Pet enum + derived status logic (P0)

GerbilStatus: Active→Breeding, add Pet; Deceased/GivenAway are now DERIVED.
GerbilStatusService.Derive(): central precedence: DateOfDeath→Deceased (1),
ReceiverContactId→GivenAway (2), age>7y→Deceased presumed (3), user choice (4).
All write paths (gerbil CRUD, contracts, importers) call GerbilStatusService.Apply().
Startup sweep flips >7y gerbils to Deceased at next app restart.
Migration StatusModel: Active→Breeding rename + backfill derived statuses.
10 new tests; 200/200 green; has-pending=No.

Enum string values: Breeding (was Active), Pet (new), Deceased, GivenAway, ForSale.
FE contract: status field values updated (see Done-Report).

GerbilManager.Tests/ExportTests.cs

@@ -60,7 +60,7 @@ namespace GerbilManager.Tests
                     Id = Guid.NewGuid(),
                     Name = "Krümel",
                     Gender = Gender.female,
-                    Status = GerbilStatus.Active,
+                    Status = GerbilStatus.Breeding,
                     DateOfBirth = new DateOnly(2025, 3, 12),
                     LitterId = litter.Id,
                     EnclosureId = enclosure.Id,
@@ -118,7 +118,7 @@ namespace GerbilManager.Tests
             var row = lines[1];
             Assert.Contains("Krümel", row);
             Assert.Contains("Weiblich", row);
-            Assert.Contains("Aktiv", row);
+            Assert.Contains("Zucht", row);
             Assert.Contains("12.03.2025", row);
             Assert.Contains("Großbecken", row);
             Assert.Contains("Wurf K", row);