feat: [시공관리] 계약관리 API 구현

- Contract 모델, 마이그레이션 추가
- ContractController CRUD 엔드포인트 구현
- ContractService 비즈니스 로직 구현
- ContractStoreRequest, ContractUpdateRequest 검증 추가
- Swagger API 문서 작성
- 라우트 등록 (GET/POST/PUT/DELETE)
- 통계 및 단계별 건수 조회 API 추가

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

app/Http/Controllers/Api/V1/Construction/ContractController.php

@@ -0,0 +1,99 @@
+<?php
+
+namespace App\Http\Controllers\Api\V1\Construction;
+
+use App\Helpers\ApiResponse;
+use App\Http\Controllers\Controller;
+use App\Http\Requests\Construction\ContractStoreRequest;
+use App\Http\Requests\Construction\ContractUpdateRequest;
+use App\Services\Construction\ContractService;
+use Illuminate\Http\Request;
+
+class ContractController extends Controller
+{
+    public function __construct(private ContractService $service) {}
+
+    /**
+     * 계약 목록 조회
+     */
+    public function index(Request $request)
+    {
+        return ApiResponse::handle(function () use ($request) {
+            return $this->service->index($request->all());
+        }, __('message.contract.fetched'));
+    }
+
+    /**
+     * 계약 상세 조회
+     */
+    public function show(int $id)
+    {
+        return ApiResponse::handle(function () use ($id) {
+            return $this->service->show($id);
+        }, __('message.contract.fetched'));
+    }
+
+    /**
+     * 계약 등록
+     */
+    public function store(ContractStoreRequest $request)
+    {
+        return ApiResponse::handle(function () use ($request) {
+            return $this->service->store($request->validated());
+        }, __('message.contract.created'));
+    }
+
+    /**
+     * 계약 수정
+     */
+    public function update(ContractUpdateRequest $request, int $id)
+    {
+        return ApiResponse::handle(function () use ($request, $id) {
+            return $this->service->update($id, $request->validated());
+        }, __('message.contract.updated'));
+    }
+
+    /**
+     * 계약 삭제
+     */
+    public function destroy(int $id)
+    {
+        return ApiResponse::handle(function () use ($id) {
+            $this->service->destroy($id);
+
+            return 'success';
+        }, __('message.contract.deleted'));
+    }
+
+    /**
+     * 계약 일괄 삭제
+     */
+    public function bulkDestroy(Request $request)
+    {
+        return ApiResponse::handle(function () use ($request) {
+            $this->service->bulkDestroy($request->input('ids', []));
+
+            return 'success';
+        }, __('message.contract.deleted'));
+    }
+
+    /**
+     * 계약 통계 조회
+     */
+    public function stats(Request $request)
+    {
+        return ApiResponse::handle(function () use ($request) {
+            return $this->service->stats($request->all());
+        }, __('message.contract.fetched'));
+    }
+
+    /**
+     * 계약 단계별 카운트 조회
+     */
+    public function stageCounts(Request $request)
+    {
+        return ApiResponse::handle(function () use ($request) {
+            return $this->service->stageCounts($request->all());
+        }, __('message.contract.fetched'));
+    }
+}