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/Models/Stats/Monthly/StatProjectMonthly.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace App\Models\Stats\Monthly;
+
+use App\Models\Stats\BaseStatModel;
+
+class StatProjectMonthly extends BaseStatModel
+{
+    protected $table = 'stat_project_monthly';
+
+    protected $casts = [
+        'contract_total_amount' => 'decimal:2',
+        'expected_expense_total' => 'decimal:2',
+        'actual_expense_total' => 'decimal:2',
+        'labor_cost_total' => 'decimal:2',
+        'material_cost_total' => 'decimal:2',
+        'gross_profit' => 'decimal:2',
+        'gross_profit_rate' => 'decimal:2',
+    ];
+}