feat(tier): Erwerb/Kauf je Tier (Datum, Preis, Notiz)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

GerbilManagerWebAPI/Migrations/20260622201453_AddAcquisitionRecord.cs

@@ -0,0 +1,44 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace GerbilManagerWebAPI.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddAcquisitionRecord : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "AcquisitionRecords",
+                columns: table => new
+                {
+                    Id = table.Column<Guid>(type: "uuid", nullable: false),
+                    GerbilId = table.Column<Guid>(type: "uuid", nullable: true),
+                    SourceContactId = table.Column<Guid>(type: "uuid", nullable: true),
+                    Date = table.Column<DateOnly>(type: "date", nullable: true),
+                    Price = table.Column<decimal>(type: "numeric(10,2)", precision: 10, scale: 2, nullable: true),
+                    Note = table.Column<string>(type: "text", nullable: true),
+                    CreatedAt = table.Column<DateTimeOffset>(type: "timestamp with time zone", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_AcquisitionRecords", x => x.Id);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_AcquisitionRecords_GerbilId",
+                table: "AcquisitionRecords",
+                column: "GerbilId");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "AcquisitionRecords");
+        }
+    }
+}