cmd/horsebot: serve data json on http

For #7.

cmd/horsebot/main.go

@@ -113,6 +113,7 @@ func main() {
 
 	mux := http.NewServeMux()
 	mux.Handle("GET /", httpmiddle.Compress(5)(http.FileServerFS(os.DirFS(public))))
+	mux.Handle("GET /api/data/", httpmiddle.Compress(5)(http.StripPrefix("/api/data", http.FileServerFS(os.DirFS(dataDir)))))
 	if pubkey != "" {
 		pk, err := hex.DecodeString(pubkey)
 		if err != nil {