Gitea migration SSRF to reserved addresses through ALLOWED_DOMAINS

With `[migrations] ALLOWED_DOMAINS` set to a matching entry such as `*` or a hostname wildcard, Gitea's migration URL validation could permit reserved and link-local addresses, such as `169.254.169.254`, even when `ALLOW_LOCALNETWORKS = false`. The local-network block list did not cover these ranges, and a hostname matching the allow list was accepted regardless of its resolved address. A user who can start migrations on such an instance could reach these addresses from the Gitea server; the default empty `ALLOWED_DOMAINS` configuration is not affected.