FEAT-14a: Charakterbogen on Gerbil (traits + note) + sale-ad request extension

- Gerbil.CharacterTraits (List<string>, JSON text column, opaque labels) + CharacterNote
  (string?). On GerbilDto + Input; round-trips on GET/POST/PUT. Additive migration.
- SaleAdAnimal gains Traits (German labels) + CharacterNote; prompt builder emits
  "Charakter: …" + "Charakter-Notiz: …" lines for the AI Verkaufstext.
- Tests: traits/note round-trip (POST->GET via SQLite host), empty-default, prompt inclusion.

GerbilManagerWebAPI/ApplicationContext.cs

@@ -49,6 +49,19 @@ public class ApplicationContext : DbContext
             e.Property(g => g.Gender).HasConversion<string>();
             e.Property(g => g.Status).HasConversion<string>();
 
+            // FEAT-14: character traits stored as a JSON text column (works on both
+            // Npgsql and the SQLite test host; opaque labels, no backend vocabulary).
+            var traitsConverter = new Microsoft.EntityFrameworkCore.Storage.ValueConversion.ValueConverter<List<string>, string>(
+                v => System.Text.Json.JsonSerializer.Serialize(v, (System.Text.Json.JsonSerializerOptions?)null),
+                v => string.IsNullOrEmpty(v)
+                    ? new List<string>()
+                    : System.Text.Json.JsonSerializer.Deserialize<List<string>>(v, (System.Text.Json.JsonSerializerOptions?)null) ?? new List<string>());
+            var traitsComparer = new Microsoft.EntityFrameworkCore.ChangeTracking.ValueComparer<List<string>>(
+                (a, b) => (a ?? new List<string>()).SequenceEqual(b ?? new List<string>()),
+                v => v == null ? 0 : v.Aggregate(0, (h, s) => HashCode.Combine(h, s.GetHashCode())),
+                v => v.ToList());
+            e.Property(g => g.CharacterTraits).HasConversion(traitsConverter, traitsComparer);
+
             e.HasOne(g => g.Litter).WithMany()
                 .HasForeignKey(g => g.LitterId).OnDelete(DeleteBehavior.SetNull);
             e.HasOne(g => g.OriginContact).WithMany()