feat: 회원가입 + 테넌트 생성 통합 API 추가 (/api/v1/register)

- 사용자 등록 + 테넌트 생성 + 시스템 관리자 권한 자동 부여
- 사업자번호 조건부 검증 (active 테넌트만 unique)
- 글로벌 메뉴 자동 복제 (parent_id 매핑 알고리즘)
- DB 트랜잭션으로 전체 프로세스 원자성 보장

추가:
- RegisterRequest: FormRequest 검증 (conditional unique)
- RegisterService: 9-step 통합 비즈니스 로직
- RegisterController: ApiResponse::handle() 패턴
- RegisterApi: 완전한 Swagger 문서

수정:
- MenusStep: 글로벌 메뉴 복제 로직 구현
- message.php: 'registered' 키 추가
- error.php: 4개 에러 메시지 추가
- routes/api.php: POST /api/v1/register 라우트

SAM API Rules 준수:
- Service-First, FormRequest, i18n, Swagger, DB Transaction

app/Http/Requests/RegisterRequest.php

@@ -0,0 +1,97 @@
+<?php
+
+namespace App\Http\Requests;
+
+use Illuminate\Foundation\Http\FormRequest;
+use Illuminate\Validation\Rule;
+
+class RegisterRequest extends FormRequest
+{
+    /**
+     * Determine if the user is authorized to make this request.
+     */
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /**
+     * Get the validation rules that apply to the request.
+     *
+     * @return array<string, \Illuminate\Contracts\Validation\ValidationRule|array<mixed>|string>
+     */
+    public function rules(): array
+    {
+        return [
+            // User fields
+            'user_id' => [
+                'required',
+                'string',
+                'max:255',
+                'regex:/^[a-zA-Z0-9_-]+$/',
+                'unique:users,user_id',
+            ],
+            'name' => 'required|string|max:255',
+            'email' => [
+                'required',
+                'string',
+                'email',
+                'max:255',
+                'unique:users,email',
+            ],
+            'phone' => [
+                'nullable',
+                'string',
+                'max:20',
+                'regex:/^[0-9-]+$/',
+            ],
+            'password' => 'required|string|min:8|confirmed',
+            'position' => 'nullable|string|max:100',
+
+            // Tenant fields
+            'company_name' => 'required|string|max:255',
+            'business_num' => [
+                'required',
+                'string',
+                'regex:/^\d{3}-\d{2}-\d{5}$/',
+                Rule::unique('tenants', 'business_num')->where(function ($query) {
+                    return $query->where('tenant_st_code', 'active');
+                }),
+            ],
+            'company_scale' => 'nullable|string|max:50',
+            'industry' => 'nullable|string|max:100',
+        ];
+    }
+
+    /**
+     * Get custom attributes for validator errors.
+     */
+    public function attributes(): array
+    {
+        return [
+            'user_id' => __('validation.attributes.user_id'),
+            'name' => __('validation.attributes.name'),
+            'email' => __('validation.attributes.email'),
+            'phone' => __('validation.attributes.phone'),
+            'password' => __('validation.attributes.password'),
+            'position' => __('validation.attributes.position'),
+            'company_name' => __('validation.attributes.company_name'),
+            'business_num' => __('validation.attributes.business_num'),
+            'company_scale' => __('validation.attributes.company_scale'),
+            'industry' => __('validation.attributes.industry'),
+        ];
+    }
+
+    /**
+     * Get custom messages for validator errors.
+     */
+    public function messages(): array
+    {
+        return [
+            'business_num.regex' => __('error.business_num_format'),
+            'business_num.unique' => __('error.business_num_duplicate_active'),
+            'user_id.regex' => __('error.user_id_format'),
+            'phone.regex' => __('error.phone_format'),
+        ];
+    }
+}