feat: 공정 단계(ProcessStep) CRUD API 구현

- process_steps 테이블 마이그레이션 생성 (step_code, sort_order, boolean 플래그 등)
- ProcessStep 모델 생성 (child entity 패턴, HasFactory만 사용)
- ProcessStepService: CRUD + reorder + STP-001 자동채번
- ProcessStepController: DI + ApiResponse::handle 패턴
- FormRequest 3개: Store, Update, Reorder
- Process 모델에 steps() HasMany 관계 추가
- ProcessService eager-load에 steps 추가 (5곳)
- Nested routes: /processes/{processId}/steps

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

app/Http/Requests/V1/ProcessStep/UpdateProcessStepRequest.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace App\Http\Requests\V1\ProcessStep;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class UpdateProcessStepRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'step_name' => ['sometimes', 'required', 'string', 'max:100'],
+            'is_required' => ['nullable', 'boolean'],
+            'needs_approval' => ['nullable', 'boolean'],
+            'needs_inspection' => ['nullable', 'boolean'],
+            'is_active' => ['nullable', 'boolean'],
+            'connection_type' => ['nullable', 'string', 'max:20'],
+            'connection_target' => ['nullable', 'string', 'max:255'],
+            'completion_type' => ['nullable', 'string', 'max:30'],
+        ];
+    }
+
+    public function attributes(): array
+    {
+        return [
+            'step_name' => '단계명',
+            'is_required' => '필수여부',
+            'needs_approval' => '승인필요여부',
+            'needs_inspection' => '검사필요여부',
+            'is_active' => '사용여부',
+            'connection_type' => '연결유형',
+            'connection_target' => '연결대상',
+            'completion_type' => '완료유형',
+        ];
+    }
+}