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/hosts.go

@@ -65,6 +65,14 @@ func (a *HostAPI) Create(w http.ResponseWriter, r *http.Request) {
 		writeJSONErr(w, http.StatusBadRequest, "unknown server_type")
 		return
 	}
+	if existing, err := a.Hosts.GetByHostname(r.Context(), req.Hostname); err == nil && existing != nil {
+		writeJSONErr(w, http.StatusConflict, "a host with this hostname already exists")
+		return
+	}
+	if existing, err := a.Hosts.GetByMAC(r.Context(), req.MAC); err == nil && existing != nil {
+		writeJSONErr(w, http.StatusConflict, "a host with this MAC address already exists")
+		return
+	}
 
 	id, err := a.Hosts.Create(r.Context(), model.Host{
 		Hostname:   req.Hostname,