fix: item_sections.page_id nullable 변경

- 섹션 템플릿(is_template=true)은 특정 페이지에 속하지 않으므로 page_id가 null 허용
- section-templates API 500 에러 해결

database/migrations/2025_11_26_170358_make_page_id_nullable_in_item_sections_table.php

@@ -0,0 +1,34 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+/**
+ * item_sections.page_id nullable 변경
+ *
+ * 목적: 섹션 템플릿(is_template=true)은 특정 페이지에 속하지 않으므로
+ *       page_id가 null일 수 있어야 함
+ */
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('item_sections', function (Blueprint $table) {
+            $table->unsignedBigInteger('page_id')->nullable()->comment('페이지 ID (템플릿은 null)')->change();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('item_sections', function (Blueprint $table) {
+            $table->unsignedBigInteger('page_id')->nullable(false)->comment('페이지 ID')->change();
+        });
+    }
+};