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).

GerbilManagerWebAPI/Export/ExportService.cs

@@ -48,9 +48,11 @@ namespace GerbilManagerWebAPI.Export
 
         private static readonly Dictionary<GerbilStatus, string> StatusDe = new()
         {
-            [GerbilStatus.Active] = "Aktiv",
+            [GerbilStatus.Breeding] = "Zucht",
             [GerbilStatus.Deceased] = "Verstorben",
             [GerbilStatus.GivenAway] = "Abgegeben",
+            [GerbilStatus.ForSale] = "Abzugeben",
+            [GerbilStatus.Pet] = "Liebhaber",
         };
 
         private static readonly Dictionary<HealthRecordType, string> HealthTypeDe = new()