feat: add GitHub Action for daily health check cron and backfill script

- Create .github/workflows/health-check.yml to run daily at 2am UTC
- Add scripts/backfill-health-checks.ts to populate health data for existing tools
- Remove health-check from vercel.json crons (now using GitHub Actions)

The GitHub Action workflow follows the same pattern as other sync operations
and calls the /api/sync/health-check endpoint with proper authentication.

The backfill script:
- Fetches all tools from database
- Runs batch health checks with concurrency control (5 tools at a time)
- Shows detailed progress and summary statistics
- Lists broken tools with error details

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

vercel.json

@@ -21,10 +21,6 @@
     {
       "path": "/api/sync/metrics",
       "schedule": "0 * * * *"
-    },
-    {
-      "path": "/api/sync/health-check",
-      "schedule": "0 2 * * *"
     }
   ]
 }