fix(domain): validate hostname format to reject invalid characters (#4729)

* fix(domain): validate hostname format to reject invalid characters

Underscores and other invalid characters were accepted in domain
inputs with no validation, causing Let's Encrypt to silently fail
certificate issuance while Dokploy fell back to a self-signed cert.

Fixes #4716

* fix(create-server): update SSH key label for clarity in server creation form

packages/server/src/utils/hostname-validation.ts

@@ -0,0 +1,8 @@
+// Valid hostname per RFC 1123: labels of letters, digits and hyphens
+// (no leading/trailing hyphen), separated by dots. Underscores are rejected
+// because Let's Encrypt refuses to issue certificates for them.
+export const VALID_HOSTNAME_REGEX =
+	/^(?:[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?\.)+[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?$/;
+
+export const INVALID_HOSTNAME_MESSAGE =
+	"Invalid domain name. Use only letters, numbers, hyphens and dots (e.g. example.com). Underscores are not allowed.";