feat: [approval] 전자결재 모듈 API 구현

- 마이그레이션 4개 (approval_forms, approval_lines, approvals, approval_steps)
- 모델 4개 (ApprovalForm, ApprovalLine, Approval, ApprovalStep)
- ApprovalService 비즈니스 로직 (양식/결재선 CRUD, 기안함/결재함/참조함, 결재 액션)
- 컨트롤러 3개 (ApprovalFormController, ApprovalLineController, ApprovalController)
- FormRequest 13개 (양식/결재선/문서 검증)
- Swagger 문서 3개 (26개 엔드포인트)
- i18n 메시지/에러 키 추가
- 라우트 26개 등록

app/Models/Tenants/ApprovalForm.php

@@ -0,0 +1,133 @@
+<?php
+
+namespace App\Models\Tenants;
+
+use App\Models\Members\User;
+use App\Traits\BelongsToTenant;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+use Illuminate\Database\Eloquent\SoftDeletes;
+
+/**
+ * 결재 양식 모델
+ *
+ * @property int $id
+ * @property int $tenant_id
+ * @property string $name
+ * @property string $code
+ * @property string|null $category
+ * @property array $template
+ * @property bool $is_active
+ * @property int|null $created_by
+ * @property int|null $updated_by
+ * @property int|null $deleted_by
+ */
+class ApprovalForm extends Model
+{
+    use BelongsToTenant, SoftDeletes;
+
+    protected $table = 'approval_forms';
+
+    protected $casts = [
+        'template' => 'array',
+        'is_active' => 'boolean',
+    ];
+
+    protected $fillable = [
+        'tenant_id',
+        'name',
+        'code',
+        'category',
+        'template',
+        'is_active',
+        'created_by',
+        'updated_by',
+        'deleted_by',
+    ];
+
+    protected $attributes = [
+        'is_active' => true,
+    ];
+
+    // =========================================================================
+    // 카테고리 상수
+    // =========================================================================
+
+    public const CATEGORY_REQUEST = 'request';           // 품의서
+
+    public const CATEGORY_EXPENSE = 'expense';           // 지출결의서
+
+    public const CATEGORY_EXPENSE_ESTIMATE = 'expense_estimate'; // 지출 예상 내역서
+
+    public const CATEGORIES = [
+        self::CATEGORY_REQUEST,
+        self::CATEGORY_EXPENSE,
+        self::CATEGORY_EXPENSE_ESTIMATE,
+    ];
+
+    // =========================================================================
+    // 관계 정의
+    // =========================================================================
+
+    /**
+     * 이 양식으로 생성된 결재 문서들
+     */
+    public function approvals(): HasMany
+    {
+        return $this->hasMany(Approval::class, 'form_id');
+    }
+
+    /**
+     * 생성자
+     */
+    public function creator(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'created_by');
+    }
+
+    /**
+     * 수정자
+     */
+    public function updater(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'updated_by');
+    }
+
+    // =========================================================================
+    // 스코프
+    // =========================================================================
+
+    /**
+     * 활성 양식만
+     */
+    public function scopeActive($query)
+    {
+        return $query->where('is_active', true);
+    }
+
+    /**
+     * 특정 카테고리
+     */
+    public function scopeCategory($query, string $category)
+    {
+        return $query->where('category', $category);
+    }
+
+    // =========================================================================
+    // 헬퍼 메서드
+    // =========================================================================
+
+    /**
+     * 카테고리 라벨
+     */
+    public function getCategoryLabelAttribute(): string
+    {
+        return match ($this->category) {
+            self::CATEGORY_REQUEST => '품의서',
+            self::CATEGORY_EXPENSE => '지출결의서',
+            self::CATEGORY_EXPENSE_ESTIMATE => '지출 예상 내역서',
+            default => $this->category ?? '',
+        };
+    }
+}