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/20260606070929_AddCharacterFields.cs

@@ -0,0 +1,39 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace GerbilManagerWebAPI.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddCharacterFields : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "CharacterNote",
+                table: "Gerbils",
+                type: "text",
+                nullable: true);
+
+            migrationBuilder.AddColumn<string>(
+                name: "CharacterTraits",
+                table: "Gerbils",
+                type: "text",
+                nullable: false,
+                defaultValue: "");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "CharacterNote",
+                table: "Gerbils");
+
+            migrationBuilder.DropColumn(
+                name: "CharacterTraits",
+                table: "Gerbils");
+        }
+    }
+}