feat(시공사): 2.1 현장관리 - Backend API 확장

- 마이그레이션: site_code, client_id, status 컬럼 추가
- Site 모델: 상태 상수, Client 관계 추가
- SiteService: stats(), bulkDestroy(), 필터 확장
- SiteController: stats, bulkDestroy 엔드포인트 추가
- 라우트: /stats, /bulk 추가

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

database/migrations/2026_01_09_162534_add_construction_fields_to_sites_table.php

@@ -0,0 +1,44 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('sites', function (Blueprint $table) {
+            // 현장코드
+            $table->string('site_code', 50)->nullable()->after('tenant_id')->comment('현장코드');
+
+            // 거래처 연결 (clients 테이블)
+            $table->foreignId('client_id')->nullable()->after('site_code')
+                ->constrained('clients')->nullOnDelete()->comment('거래처 ID');
+
+            // 상태 (is_active와 별개)
+            $table->enum('status', ['unregistered', 'suspended', 'active', 'pending'])
+                ->default('unregistered')->after('is_active')->comment('상태: 미등록/중지/사용/보류');
+
+            // 인덱스
+            $table->index(['tenant_id', 'site_code']);
+            $table->index(['tenant_id', 'status']);
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('sites', function (Blueprint $table) {
+            $table->dropIndex(['tenant_id', 'site_code']);
+            $table->dropIndex(['tenant_id', 'status']);
+            $table->dropForeign(['client_id']);
+            $table->dropColumn(['site_code', 'client_id', 'status']);
+        });
+    }
+};