feat: sam_stat 통계 DB 인프라 구축 (Phase 1)

- sam_stat DB 연결 설정 (config/database.php, .env)
- 메타 테이블 마이그레이션 (stat_definitions, stat_job_logs)
- dim_date 차원 테이블 + DimDateSeeder (2020~2030, 4018건)
- 기반 모델: BaseStatModel, StatDefinition, StatJobLog, DimDate
- 집계 커맨드: stat:aggregate-daily, stat:aggregate-monthly
- StatAggregatorService + StatDomainServiceInterface 골격

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

app/Models/Stats/StatDefinition.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace App\Models\Stats;
+
+class StatDefinition extends BaseStatModel
+{
+    protected $table = 'stat_definitions';
+
+    protected $casts = [
+        'source_tables' => 'array',
+        'config' => 'array',
+        'is_active' => 'boolean',
+    ];
+}