feat: sam_stat P2 도메인 + 통계 API + 대시보드 전환 (Phase 4)

- 4.1: stat_project_monthly + ProjectStatService (건설/프로젝트 월간)
- 4.2: stat_system_daily + SystemStatService (API/감사/FCM/파일/결재)
- 4.3: stat_events, stat_snapshots + StatEventService + StatEventObserver
- 4.4: StatController (summary/daily/monthly/alerts) + StatQueryService + FormRequest 3개 + routes/stats.php
- 4.5: DashboardService sam_stat 우선 조회 + 원본 DB 폴백 패턴

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/Http/Requests/V1/Stat/StatAlertRequest.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace App\Http\Requests\V1\Stat;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class StatAlertRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'limit' => 'nullable|integer|min:1|max:100',
+            'unread_only' => 'nullable|boolean',
+        ];
+    }
+}