* Fix startup message and add connectivity check for S3 storage
When S3 storage is configured, the startup log incorrectly showed the
default local path (./cache/artifacts) instead of the actual S3 URL.
This also adds a lightweight connectivity check at startup so bad
credentials or endpoints fail immediately rather than on first request.
Add URL() and Close() to the Storage interface so all backends report
their URL and can be cleaned up properly. Rename the stats JSON field
from storage_path to storage_url. Close storage in error paths and
during graceful shutdown.
Fixes#49
* Fix Windows test assertion for file:// URL format
OpenBucket normalizes Windows paths to file:///C:/path (three slashes)
but the test expected file://C:/path (two slashes).
