feat: add historical stats tracking with daily snapshots

- Add StatsSnapshot model to store daily registry metrics
- Create /api/sync/stats-snapshot endpoint for daily cron captures
- Add GET endpoint to retrieve historical snapshots (up to 365 days)
- Update stats page with Historical Trends section showing:
  - Tools & packages growth over time
  - Health status trends
  - Daily executions history
  - NPM downloads trends
- Add cron job running at midnight UTC daily
- Fix PostgreSQL GROUP BY issue in quality distribution query

vercel.json

@@ -48,6 +48,10 @@
     {
       "path": "/api/sync/metrics",
       "schedule": "0 * * * *"
+    },
+    {
+      "path": "/api/sync/stats-snapshot",
+      "schedule": "0 0 * * *"
     }
   ]
 }