feat(API): 입금/출금 알림 Observer 추가 및 LoanController 수정

- DepositIssueObserver, WithdrawalIssueObserver 신규 추가
- TodayIssueObserverService에 입금/출금 핸들러 및 디버그 로그 추가
- TodayIssue 모델에 입금/출금 상수 추가
- AppServiceProvider에 Observer 등록
- ApprovalService에 기존 결재선 사용 시 수동 알림 트리거 추가
- LoanController ApiResponse::handle() → ApiResponse::success() 수정

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

app/Observers/TodayIssue/DepositIssueObserver.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace App\Observers\TodayIssue;
+
+use App\Models\Tenants\Deposit;
+use App\Services\TodayIssueObserverService;
+use Illuminate\Support\Facades\Log;
+
+/**
+ * Deposit 모델의 TodayIssue Observer
+ */
+class DepositIssueObserver
+{
+    public function __construct(
+        protected TodayIssueObserverService $service
+    ) {}
+
+    public function created(Deposit $deposit): void
+    {
+        Log::info('DepositIssueObserver::created CALLED', [
+            'deposit_id' => $deposit->id,
+            'tenant_id' => $deposit->tenant_id,
+        ]);
+
+        $this->safeExecute(fn () => $this->service->handleDepositCreated($deposit));
+    }
+
+    public function deleted(Deposit $deposit): void
+    {
+        $this->safeExecute(fn () => $this->service->handleDepositDeleted($deposit));
+    }
+
+    protected function safeExecute(callable $callback): void
+    {
+        try {
+            $callback();
+        } catch (\Throwable $e) {
+            Log::warning('TodayIssue DepositObserver failed', [
+                'error' => $e->getMessage(),
+            ]);
+        }
+    }
+}