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).

GerbilManager.Tests/PartialUpdateTests.cs

@@ -91,7 +91,7 @@ public class PartialUpdateTests : IClassFixture<ApiFactory>
         // Assert: name / genotype / originBreeder / status / externalRef all intact
         var json = JsonDocument.Parse(await _client.GetStringAsync($"/gerbils/{id}")).RootElement;
 
-        Assert.Equal("Active", GetStr(json, "status"));
+        Assert.Equal("Breeding", GetStr(json, "status"));
         Assert.Equal("Charakter-Tier", GetStr(json, "name"));
         Assert.Equal("female", GetStr(json, "gender"));
         Assert.Equal("aa CC DD ee GG PP spsp rere", GetStr(json, "genotype"));
@@ -138,7 +138,7 @@ public class PartialUpdateTests : IClassFixture<ApiFactory>
 
         var json = JsonDocument.Parse(await resp.Content.ReadAsStringAsync()).RootElement;
         Assert.Equal("Minimal", GetStr(json, "name"));
-        Assert.Equal("Active", GetStr(json, "status"));
+        Assert.Equal("Breeding", GetStr(json, "status"));
         // characterTraits defaults to [] on create when not provided
         Assert.Contains("[]", GetNested(json, "characterTraits") ?? "");
     }