Add CORS middleware

Fixes: https://todo.xenrox.net/~xenrox/ntfy-alertmanager/21

main.go

@@ -402,6 +402,16 @@ func (br *bridge) handleWebhooks(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+func (br *bridge) corsMiddleware(handler http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Access-Control-Allow-Origin", "*")
+		w.Header().Set("Access-Control-Allow-Methods", "POST, OPTIONS")
+		w.Header().Set("Access-Control-Allow-Headers", "Content-Type, Authorization")
+
+		handler.ServeHTTP(w, r)
+	})
+}
+
 func (br *bridge) authMiddleware(handler http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		logger := br.logger.With(slog.String("url", r.URL.String()))
@@ -495,6 +505,8 @@ func main() {
 		httpServer.Handler = bridge.authMiddleware(mux)
 	}
 
+	httpServer.Handler = bridge.corsMiddleware(httpServer.Handler)
+
 	if _, ok := c.(*cache.MemoryCache); ok {
 		go bridge.runCleanup(ctx)
 	}