fix:트리거 감사로그 created_at 단독 인덱스 추가

- ORDER BY created_at DESC 쿼리 성능 개선
- 기존 복합 인덱스만으로는 단독 정렬에 활용 불가

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

database/migrations/2026_02_19_100000_add_created_at_index_to_trigger_audit_logs.php

@@ -0,0 +1,20 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\DB;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        DB::statement("
+            CREATE INDEX ix_trig_created_at
+                ON trigger_audit_logs (created_at)
+        ");
+    }
+
+    public function down(): void
+    {
+        DB::statement('DROP INDEX ix_trig_created_at ON trigger_audit_logs');
+    }
+};