feat: [공정관리] parent_id 트리 구조 도입 — 마이그레이션, 모델 관계, 2depth 검증

database/migrations/2026_03_21_152057_add_parent_id_to_processes_table.php

@@ -0,0 +1,34 @@
+<?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('processes', function (Blueprint $table) {
+            $table->unsignedBigInteger('parent_id')
+                ->nullable()
+                ->after('tenant_id')
+                ->comment('부모 공정 ID (NULL이면 루트)');
+
+            $table->foreign('parent_id')
+                ->references('id')
+                ->on('processes')
+                ->onDelete('set null');
+
+            $table->index(['tenant_id', 'parent_id']);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('processes', function (Blueprint $table) {
+            $table->dropForeign(['parent_id']);
+            $table->dropIndex(['tenant_id', 'parent_id']);
+            $table->dropColumn('parent_id');
+        });
+    }
+};