Reuse the existing SafeWebhookUrl rule on the S3 Storage endpoint field so
the create and edit forms go through the same URL-normalization path as
webhook settings. Adds a matching guard inside S3Storage::testConnection()
so background callers (scheduled backups, database import reuse) also
validate the endpoint before building the S3 client.
Also fixes an IPv6-bracket edge case in SafeWebhookUrl so `http://[::1]`
style hosts are normalized before the filter_var IP check — the rule's
own loopback test was already asserting this behaviour.
