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)

routes/api.php

@@ -398,6 +398,7 @@
             Route::post('', [LoanController::class, 'store'])->name('v1.loans.store');
             Route::get('/summary', [LoanController::class, 'summary'])->name('v1.loans.summary');
             Route::post('/calculate-interest', [LoanController::class, 'calculateInterest'])->name('v1.loans.calculate-interest');
+            Route::get('/interest-report/{year}', [LoanController::class, 'interestReport'])->whereNumber('year')->name('v1.loans.interest-report');
             Route::get('/{id}', [LoanController::class, 'show'])->whereNumber('id')->name('v1.loans.show');
             Route::put('/{id}', [LoanController::class, 'update'])->whereNumber('id')->name('v1.loans.update');
             Route::delete('/{id}', [LoanController::class, 'destroy'])->whereNumber('id')->name('v1.loans.destroy');