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/Migrations/ApplicationContextModelSnapshot.cs

@@ -661,6 +661,13 @@ namespace GerbilManagerWebAPI.Migrations
                     b.Property<string>("CauseOfDeath")
                         .HasColumnType("text");
 
+                    b.Property<string>("CharacterNote")
+                        .HasColumnType("text");
+
+                    b.Property<string>("CharacterTraits")
+                        .IsRequired()
+                        .HasColumnType("text");
+
                     b.Property<Guid?>("ColorVarietyId")
                         .HasColumnType("uuid");