feat: item_fields에 field_key, is_locked 컬럼 추가

- field_key: {ID}_{key} 형식으로 고유키 생성
- is_locked, locked_by, locked_at 잠금 컬럼 추가
- ItemFieldService: store/update/clone 로직 수정
- FormRequest: field_key 검증 규칙 추가
- Swagger 스키마 업데이트

database/migrations/2025_11_28_171952_add_field_key_and_lock_columns_to_item_fields_table.php

@@ -0,0 +1,46 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+/**
+ * item_fields 테이블에 field_key 및 잠금 컬럼 추가
+ *
+ * 변경 내용:
+ * 1. field_key: 필드 고유 식별자 ({ID}_{key} 형식, unique)
+ * 2. is_locked: 엔티티 자체 잠금 여부
+ * 3. locked_by: 잠금 설정자 ID
+ * 4. locked_at: 잠금 설정 일시
+ */
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('item_fields', function (Blueprint $table) {
+            $table->string('field_key', 100)->nullable()->after('field_name')->comment('필드 고유 키 ({ID}_{key} 형식)');
+            $table->boolean('is_locked')->default(false)->after('is_common')->comment('잠금 여부');
+            $table->unsignedBigInteger('locked_by')->nullable()->after('is_locked')->comment('잠금 설정자 ID');
+            $table->timestamp('locked_at')->nullable()->after('locked_by')->comment('잠금 설정 일시');
+
+            // 인덱스
+            $table->unique(['tenant_id', 'field_key'], 'uq_item_fields_tenant_field_key');
+            $table->index('is_locked', 'idx_item_fields_is_locked');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('item_fields', function (Blueprint $table) {
+            $table->dropUnique('uq_item_fields_tenant_field_key');
+            $table->dropIndex('idx_item_fields_is_locked');
+            $table->dropColumn(['field_key', 'is_locked', 'locked_by', 'locked_at']);
+        });
+    }
+};