S3 backup uploads fail when the endpoint has an AAAA record: mc parses
--resolve values with netip.ParseAddr and rejects the bracketed IPv6
form. Its resolver map also keeps only one IP per host, so emit a single
entry, preferring IPv4, without brackets.
Add SafeWebhookUrl validation rule to notification webhook URL fields
(Slack, Discord, custom webhook) to enforce safe URL patterns including
scheme validation and hostname checks.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
