fix: [attendance] 주간 근무시간 계산 UNSIGNED 오버플로우 수정

- CAST(... AS UNSIGNED)에서 음수값 시 2^64-1로 오버플로우되던 버그
- CAST(... AS SIGNED) + COALESCE + GREATEST(0, ...)로 안전하게 변경
- getOvertimeAlerts(), getEmployeeMonthlySummary() 두 곳 수정

app/Services/HR/AttendanceService.php

@@ -357,7 +357,7 @@ public function getEmployeeMonthlySummary(int $year, int $month): array
                 'user_id',
                 'status',
                 DB::raw('COUNT(*) as cnt'),
-                DB::raw("SUM(CAST(JSON_UNQUOTE(JSON_EXTRACT(json_details, '$.work_minutes')) AS UNSIGNED)) as total_minutes")
+                DB::raw("SUM(GREATEST(0, COALESCE(CAST(JSON_UNQUOTE(JSON_EXTRACT(json_details, '$.work_minutes')) AS SIGNED), 0))) as total_minutes")
             )
             ->groupBy('user_id', 'status')
             ->get();
@@ -409,7 +409,7 @@ public function getOvertimeAlerts(): array
             ->betweenDates($weekStart, $weekEnd)
             ->select(
                 'user_id',
-                DB::raw("SUM(CAST(JSON_UNQUOTE(JSON_EXTRACT(json_details, '$.work_minutes')) AS UNSIGNED)) as week_minutes")
+                DB::raw("SUM(GREATEST(0, COALESCE(CAST(JSON_UNQUOTE(JSON_EXTRACT(json_details, '$.work_minutes')) AS SIGNED), 0))) as week_minutes")
             )
             ->groupBy('user_id')
             ->having('week_minutes', '>=', 2880) // 48시간 = 2880분