feat: 가지급금 관리 API 구현

- loans 테이블 마이그레이션 추가
- Loan 모델 (인정이자 계산, 세금 계산 로직)
- LoanService (CRUD, 정산, 인정이자 계산/리포트)
- LoanController, FormRequest 5개
- 9개 API 라우트 등록
- i18n 키 추가 (validation)

app/Http/Controllers/Api/V1/LoanController.php

@@ -0,0 +1,115 @@
+<?php
+
+namespace App\Http\Controllers\Api\V1;
+
+use App\Http\Controllers\Controller;
+use App\Http\Requests\Loan\LoanCalculateInterestRequest;
+use App\Http\Requests\Loan\LoanIndexRequest;
+use App\Http\Requests\Loan\LoanSettleRequest;
+use App\Http\Requests\Loan\LoanStoreRequest;
+use App\Http\Requests\Loan\LoanUpdateRequest;
+use App\Http\Response\ApiResponse;
+use App\Services\LoanService;
+use Illuminate\Http\JsonResponse;
+
+class LoanController extends Controller
+{
+    public function __construct(
+        private readonly LoanService $loanService
+    ) {}
+
+    /**
+     * 가지급금 목록
+     */
+    public function index(LoanIndexRequest $request): JsonResponse
+    {
+        $result = $this->loanService->index($request->validated());
+
+        return ApiResponse::handle('message.fetched', $result);
+    }
+
+    /**
+     * 가지급금 요약
+     */
+    public function summary(LoanIndexRequest $request): JsonResponse
+    {
+        $userId = $request->validated()['user_id'] ?? null;
+        $result = $this->loanService->summary($userId);
+
+        return ApiResponse::handle('message.fetched', $result);
+    }
+
+    /**
+     * 가지급금 등록
+     */
+    public function store(LoanStoreRequest $request): JsonResponse
+    {
+        $result = $this->loanService->store($request->validated());
+
+        return ApiResponse::handle('message.created', $result, 201);
+    }
+
+    /**
+     * 가지급금 상세
+     */
+    public function show(int $id): JsonResponse
+    {
+        $result = $this->loanService->show($id);
+
+        return ApiResponse::handle('message.fetched', $result);
+    }
+
+    /**
+     * 가지급금 수정
+     */
+    public function update(LoanUpdateRequest $request, int $id): JsonResponse
+    {
+        $result = $this->loanService->update($id, $request->validated());
+
+        return ApiResponse::handle('message.updated', $result);
+    }
+
+    /**
+     * 가지급금 삭제
+     */
+    public function destroy(int $id): JsonResponse
+    {
+        $this->loanService->destroy($id);
+
+        return ApiResponse::handle('message.deleted');
+    }
+
+    /**
+     * 가지급금 정산
+     */
+    public function settle(LoanSettleRequest $request, int $id): JsonResponse
+    {
+        $result = $this->loanService->settle($id, $request->validated());
+
+        return ApiResponse::handle('message.loan.settled', $result);
+    }
+
+    /**
+     * 인정이자 계산
+     */
+    public function calculateInterest(LoanCalculateInterestRequest $request): JsonResponse
+    {
+        $validated = $request->validated();
+        $result = $this->loanService->calculateInterest(
+            $validated['year'],
+            $validated['user_id'] ?? null
+        );
+
+        return ApiResponse::handle('message.fetched', $result);
+    }
+
+    /**
+     * 인정이자 리포트
+     */
+    public function interestReport(int $year): JsonResponse
+    {
+        $result = $this->loanService->interestReport($year);
+
+        return ApiResponse::handle('message.fetched', $result);
+    }
+}