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>

config/database.php

@@ -62,6 +62,26 @@
             ]) : [],
         ],
 
+        // 통계 전용 DB (sam_stat)
+        'sam_stat' => [
+            'driver' => 'mysql',
+            'host' => env('STAT_DB_HOST', env('DB_HOST', '127.0.0.1')),
+            'port' => env('STAT_DB_PORT', env('DB_PORT', '3306')),
+            'database' => env('STAT_DB_DATABASE', 'sam_stat'),
+            'username' => env('STAT_DB_USERNAME', env('DB_USERNAME', 'root')),
+            'password' => env('STAT_DB_PASSWORD', env('DB_PASSWORD', '')),
+            'unix_socket' => env('DB_SOCKET', ''),
+            'charset' => 'utf8mb4',
+            'collation' => 'utf8mb4_unicode_ci',
+            'prefix' => '',
+            'prefix_indexes' => true,
+            'strict' => true,
+            'engine' => null,
+            'options' => extension_loaded('pdo_mysql') ? array_filter([
+                PDO::MYSQL_ATTR_SSL_CA => env('MYSQL_ATTR_SSL_CA'),
+            ]) : [],
+        ],
+
         // 5130 레거시 DB (chandj)
         'chandj' => [
             'driver' => 'mysql',