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/Services/GerbilStatusService.cs

@@ -0,0 +1,53 @@
+using GerbilManagerWebAPI.Models;
+
+namespace GerbilManagerWebAPI.Services
+{
+    /// <summary>
+    /// Central status-derivation logic. Every write path (create/update gerbil, SaleContract
+    /// Abgabe, import, startup sweep) calls Apply() after setting the other fields so the
+    /// derived statuses (Deceased, GivenAway) are always consistent.
+    ///
+    /// Precedence (highest wins):
+    ///   1) DateOfDeath set           → Deceased (explicit, always)
+    ///   2) Abgabe (ReceiverContactId set) → GivenAway
+    ///   3) Age > MaxAgeYears without a death date or Abgabe → Deceased (presumed)
+    ///   4) User-supplied {Breeding, Pet, ForSale}; defaults Breeding if invalid
+    ///
+    /// Age-based death is time-dependent. The stored column is kept up-to-date by:
+    ///   a) Apply() on every write (catches the animal at write time)
+    ///   b) A startup sweep in Program.cs (catches animals that silently crossed the threshold)
+    /// Gridify filters on the stored value, so status==Breeding never surfaces >7y animals.
+    /// </summary>
+    public static class GerbilStatusService
+    {
+        public const int MaxAgeYears = 7;
+
+        /// <summary>Derives and sets g.Status using the gerbil's current field values.
+        /// Must be called AFTER all other fields (DateOfDeath, ReceiverContactId, DateOfBirth)
+        /// have been applied. today = DateOnly.FromDateTime(DateTime.UtcNow).</summary>
+        public static void Apply(Gerbil g, DateOnly today)
+        {
+            g.Status = Derive(g.Status, g.DateOfBirth, g.DateOfDeath,
+                isAbgegeben: g.ReceiverContactId is not null, today);
+        }
+
+        /// <summary>Pure derivation — useful for tests and the migration backfill.</summary>
+        public static GerbilStatus Derive(
+            GerbilStatus requested,
+            DateOnly? dateOfBirth,
+            DateOnly? dateOfDeath,
+            bool isAbgegeben,
+            DateOnly today)
+        {
+            if (dateOfDeath is not null) return GerbilStatus.Deceased;
+            if (isAbgegeben) return GerbilStatus.GivenAway;
+            if (dateOfBirth is not null && IsOlderThan(dateOfBirth.Value, MaxAgeYears, today))
+                return GerbilStatus.Deceased;
+            return requested is GerbilStatus.Breeding or GerbilStatus.Pet or GerbilStatus.ForSale
+                ? requested : GerbilStatus.Breeding;
+        }
+
+        private static bool IsOlderThan(DateOnly dob, int years, DateOnly today) =>
+            today >= dob.AddYears(years);
+    }
+}