feat: [interview] 카테고리 계층 구조 parent_id 마이그레이션 추가

- interview_categories 테이블에 parent_id 컬럼 추가
- self-referencing FK, nullOnDelete

database/migrations/2026_02_28_100005_add_parent_id_to_interview_categories_table.php

@@ -0,0 +1,26 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('interview_categories', function (Blueprint $table) {
+            $table->unsignedBigInteger('parent_id')->nullable()->after('interview_project_id')->comment('부모 카테고리 ID (대분류→중분류)');
+            $table->index('parent_id', 'idx_interview_categories_parent');
+            $table->foreign('parent_id')->references('id')->on('interview_categories')->nullOnDelete();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('interview_categories', function (Blueprint $table) {
+            $table->dropForeign(['parent_id']);
+            $table->dropIndex('idx_interview_categories_parent');
+            $table->dropColumn('parent_id');
+        });
+    }
+};