Gitea migration SSRF through ALLOWED_DOMAINS address check bypass

When `[migrations] ALLOWED_DOMAINS` was configured, a hostname matching the allow list was accepted without checking its resolved address against the local-network restrictions. A user who can start repository migrations and control the DNS of an allowed hostname could make it resolve to loopback or private addresses and bypass `ALLOW_LOCALNETWORKS = false`, reaching internal services from the Gitea server. Instances without `ALLOWED_DOMAINS` configured are not affected by this specific bypass.