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/Models/Gerbil.cs

@@ -13,7 +13,7 @@ namespace GerbilManagerWebAPI.Models
         public Guid Id { get; set; }
         public required string Name { get; set; }
         public Gender Gender { get; set; }
-        public GerbilStatus Status { get; set; } = GerbilStatus.Active;
+        public GerbilStatus Status { get; set; } = GerbilStatus.Breeding;
 
         // Birth litter (the litter this gerbil was born in).
         public Guid? LitterId { get; set; }