feat: [approval] 결재 수신함 날짜 범위 필터 추가

- InboxIndexRequest에 start_date/end_date 검증 룰 추가
- ApprovalService.inbox()에 created_at 날짜 범위 필터 구현

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Http/Requests/Approval/InboxIndexRequest.php

@@ -22,6 +22,8 @@ public function rules(): array
             'sort_dir' => 'nullable|string|in:asc,desc',
             'per_page' => 'nullable|integer|min:1',
             'page' => 'nullable|integer|min:1',
+            'start_date' => 'nullable|date_format:Y-m-d',
+            'end_date' => 'nullable|date_format:Y-m-d|after_or_equal:start_date',
         ];
     }
 }