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/Services/ProcessService.php

@@ -25,7 +25,7 @@ public function index(array $params)
 
         $query = Process::query()
             ->where('tenant_id', $tenantId)
-            ->with(['classificationRules', 'processItems.item:id,code,name']);
+            ->with(['classificationRules', 'processItems.item:id,code,name', 'steps']);
 
         // 검색어
         if ($q !== '') {
@@ -62,7 +62,7 @@ public function show(int $id)
         $tenantId = $this->tenantId();
 
         $process = Process::where('tenant_id', $tenantId)
-            ->with(['classificationRules', 'processItems.item:id,code,name'])
+            ->with(['classificationRules', 'processItems.item:id,code,name', 'steps'])
             ->find($id);
 
         if (! $process) {
@@ -104,7 +104,7 @@ public function store(array $data)
             // 개별 품목 연결
             $this->syncProcessItems($process, $itemIds);
 
-            return $process->load(['classificationRules', 'processItems.item:id,code,name']);
+            return $process->load(['classificationRules', 'processItems.item:id,code,name', 'steps']);
         });
     }
 
@@ -145,7 +145,7 @@ public function update(int $id, array $data)
                 $this->syncProcessItems($process, $itemIds);
             }
 
-            return $process->fresh(['classificationRules', 'processItems.item:id,code,name']);
+            return $process->fresh(['classificationRules', 'processItems.item:id,code,name', 'steps']);
         });
     }
 
@@ -201,7 +201,7 @@ public function toggleActive(int $id)
             'updated_by' => $userId,
         ]);
 
-        return $process->fresh(['classificationRules', 'processItems.item:id,code,name']);
+        return $process->fresh(['classificationRules', 'processItems.item:id,code,name', 'steps']);
     }
 
     /**