Fix host resolution and concurrent cache updates

internal/netbox/client.go

@@ -351,6 +351,9 @@ func stripPrefix(cidr string) string {
 	return cidr
 }
 
+// ErrHostNotFound distinguishes a missing exact match from API failures or ambiguity.
+var ErrHostNotFound = errors.New("host not found")
+
 // FindHost refuses ambiguous names so writes cannot target an arbitrary server.
 func (c *Client) FindHost(ctx context.Context, name, kind string) (HostEntry, error) {
 	if kind != "" && kind != "device" && kind != "vm" {
@@ -366,6 +369,9 @@ func (c *Client) FindHost(ctx context.Context, name, kind string) (HostEntry, er
 			matches = append(matches, e)
 		}
 	}
+	if len(matches) == 0 {
+		return HostEntry{}, fmt.Errorf("%w: %q", ErrHostNotFound, name)
+	}
 	if len(matches) != 1 {
 		return HostEntry{}, fmt.Errorf("host %q: found %d exact matches; use --kind or a unique hostname", name, len(matches))
 	}