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/Import/ImportService.cs

@@ -1,6 +1,7 @@
 using System.Text.Json;
 using System.Text.RegularExpressions;
 using GerbilManagerWebAPI.Models;
+using GerbilManagerWebAPI.Services;
 using Microsoft.EntityFrameworkCore;
 
 namespace GerbilManagerWebAPI.Import
@@ -390,12 +391,12 @@ namespace GerbilManagerWebAPI.Import
 
                 if (execute)
                 {
-                    _db.Gerbils.Add(new Gerbil
+                    var importedGerbil = new Gerbil
                     {
                         Id = p.Gid,
                         Name = p.A.Name,
                         Gender = p.Gender,
-                        Status = GerbilStatus.Active,
+                        Status = GerbilStatus.Breeding,
                         DateOfBirth = ParseDate(p.A.Dob),
                         DateOfDeath = ParseDate(p.A.Death),
                         LitterId = litterId,
@@ -419,7 +420,9 @@ namespace GerbilManagerWebAPI.Import
                             p.A.SourceFiles,
                             FarbschlagRaw = p.A.Farbschlag,
                         }),
-                    });
+                    };
+                    GerbilStatusService.Apply(importedGerbil, DateOnly.FromDateTime(DateTime.UtcNow));
+                    _db.Gerbils.Add(importedGerbil);
                 }
 
                 // photos