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/ImportDocxService.cs

@@ -1,5 +1,6 @@
 using System.Text.Json;
 using GerbilManagerWebAPI.Models;
+using GerbilManagerWebAPI.Services;
 using Microsoft.EntityFrameworkCore;
 
 namespace GerbilManagerWebAPI.Import
@@ -266,7 +267,7 @@ namespace GerbilManagerWebAPI.Import
                             Name              = da.Name.Trim(),
                             DateOfBirth       = animalDob,
                             Gender            = ParseGender(da.Gender),
-                            Status            = deathDate is not null ? GerbilStatus.Deceased : GerbilStatus.GivenAway,
+                            Status            = GerbilStatusService.Derive(GerbilStatus.GivenAway, animalDob, deathDate, isAbgegeben: receiverId is not null, DateOnly.FromDateTime(DateTime.UtcNow)),
                             LitterId          = litterId,
                             ReceiverContactId = receiverId,
                             GoHomeDate        = goHomeDate,