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/Models/Process.php

@@ -69,6 +69,14 @@ public function items(): BelongsToMany
             ->orderByPivot('priority');
     }
 
+    /**
+     * 공정 단계
+     */
+    public function steps(): HasMany
+    {
+        return $this->hasMany(ProcessStep::class)->orderBy('sort_order');
+    }
+
     /**
      * 작업지시들
      */

app/Models/ProcessStep.php

@@ -0,0 +1,42 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class ProcessStep extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'process_id',
+        'step_code',
+        'step_name',
+        'is_required',
+        'needs_approval',
+        'needs_inspection',
+        'is_active',
+        'sort_order',
+        'connection_type',
+        'connection_target',
+        'completion_type',
+    ];
+
+    protected $casts = [
+        'is_required' => 'boolean',
+        'needs_approval' => 'boolean',
+        'needs_inspection' => 'boolean',
+        'is_active' => 'boolean',
+        'sort_order' => 'integer',
+    ];
+
+    /**
+     * 공정
+     */
+    public function process(): BelongsTo
+    {
+        return $this->belongsTo(Process::class);
+    }
+}