fix: use Crawlee dataset.drop() for graceful storage cleanup

[younglim]

Summary

• Replace platform-specific EPERM workaround in generateArtifacts with Crawlee's native dataset.drop() and requestQueue.drop() APIs
• Call Configuration.getStorageClient().teardown() to flush pending background writes before dropping storage
• Eliminate the 3-5 second delay and Windows retry logic that was needed to work around file locks

Test plan

• Run a full scan on macOS/Linux — verify crawlee folder is cleanly removed with no warnings
• Run on Windows — verify no EPERM errors or Node crashes
• Verify scan results (HTML report, CSV, JSON) are generated correctly
• Test error path: kill scan mid-way, verify cleanUp() doesn't hang
• Run existing test suite: npm test