Remove unused hostname_prefix from server types and add duplicate checking

The HostnamePrefix field on ServerType was loaded from YAML but never used —
hostnames are user-provided. This removes the field and adds explicit
duplicate checks (hostname + MAC) with clear per-field error messages in
both the JSON API and web UI, backed by a new GetByHostname store method
with case-insensitive matching.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/api/ui.go

@@ -74,6 +74,22 @@ func (u *UI) CreateHost(w http.ResponseWriter, r *http.Request) {
 		}))
 		return
 	}
+	if existing, err := u.Hosts.GetByHostname(r.Context(), hostname); err == nil && existing != nil {
+		errs = append(errs, "A host with this hostname already exists")
+	}
+	if existing, err := u.Hosts.GetByMAC(r.Context(), mac); err == nil && existing != nil {
+		errs = append(errs, "A host with this MAC address already exists")
+	}
+	if len(errs) > 0 {
+		types := u.ServerTypes.Keys()
+		renderHTML(w, hostFormPage(types, strings.Join(errs, "; "), &model.Host{
+			Hostname:   hostname,
+			MAC:        mac,
+			ServerType: serverType,
+			Notes:      notes,
+		}))
+		return
+	}
 
 	_, err := u.Hosts.Create(r.Context(), model.Host{
 		Hostname:   hostname,