fix:sales_contract_products.tenant_id nullable 변경

- 가망고객(prospect) 모드에서 계약상품 저장 지원
- tenant_id가 없어도 management_id로 연결 가능

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

database/migrations/2026_01_31_204350_alter_sales_contract_products_tenant_id_nullable.php

@@ -0,0 +1,29 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('sales_contract_products', function (Blueprint $table) {
+            // tenant_id를 nullable로 변경 (가망고객 모드 지원)
+            $table->unsignedBigInteger('tenant_id')->nullable()->change();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('sales_contract_products', function (Blueprint $table) {
+            $table->unsignedBigInteger('tenant_id')->nullable(false)->change();
+        });
+    }
+};