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/Endpoints/ContractEndpoints.cs

@@ -2,6 +2,7 @@ using GerbilManagerWebAPI.Common;
 using GerbilManagerWebAPI.Contracts;
 using GerbilManagerWebAPI.Dtos;
 using GerbilManagerWebAPI.Models;
+using GerbilManagerWebAPI.Services;
 using Microsoft.AspNetCore.Http.HttpResults;
 using Microsoft.EntityFrameworkCore;
 
@@ -104,11 +105,12 @@ namespace GerbilManagerWebAPI.Endpoints
                 db.SaleContracts.Add(entity);
 
                 // Abgabe-Abschluss-Semantik: in derselben SaveChanges-Transaktion.
+                var today = DateOnly.FromDateTime(DateTime.UtcNow);
                 foreach (var g in gerbils)
                 {
                     g.ReceiverContactId = contact.Id;
                     g.GoHomeDate = input.HandoverDate;
-                    g.Status = GerbilStatus.GivenAway;
+                    GerbilStatusService.Apply(g, today);
                 }
 
                 try