Make the topic feeds work with the ntfy apps

The JSON stream now carries the event field the apps look for, sends open and keepalive
events, and supports ?poll=1 and ?since= with a duration or a message id, which is how the
apps catch up. Adds a walkthrough for the phone app to the README.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/sandbar/notify.go

@@ -25,8 +25,9 @@ import (
 
 type Message struct {
 	ID       string   `json:"id"`
-	Topic    string   `json:"topic"`
 	Time     int64    `json:"time"`
+	Event    string   `json:"event"` // always "message"; the ntfy apps ignore lines without it
+	Topic    string   `json:"topic"`
 	Title    string   `json:"title,omitempty"`
 	Message  string   `json:"message"`
 	Priority int      `json:"priority"`