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/sandbar_test.go

@@ -453,6 +453,56 @@ func TestAPIAuthSetupAndTopics(t *testing.T) {
 		t.Fatal("subscriber got nothing")
 	}
 
+	// The ntfy apps catch up with ?poll=1&since=<last id> and ignore lines without "event".
+	var ids []string
+	for _, body := range []string{"one", "two", "three"} {
+		r, _ := http.NewRequest("POST", ts.URL+"/phone", strings.NewReader(body))
+		r.Header.Set("Authorization", "Bearer "+key["token"])
+		resp, err := http.DefaultClient.Do(r)
+		if err != nil || resp.StatusCode != 200 {
+			t.Fatalf("publish %s: %v %v", body, err, resp)
+		}
+		var pm Message
+		json.NewDecoder(resp.Body).Decode(&pm)
+		resp.Body.Close()
+		if pm.Event != "message" {
+			t.Fatalf("publish response event: %q", pm.Event)
+		}
+		ids = append(ids, pm.ID)
+	}
+	pollLines := func(query string) []Message {
+		req, _ := http.NewRequest("GET", ts.URL+"/phone/json?poll=1&"+query, nil)
+		req.Header.Set("Authorization", "Bearer "+key["token"])
+		resp, err := http.DefaultClient.Do(req)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer resp.Body.Close()
+		var got []Message
+		sc := bufio.NewScanner(resp.Body)
+		for sc.Scan() {
+			var m Message
+			json.Unmarshal(sc.Bytes(), &m)
+			if m.Event != "message" {
+				t.Fatalf("poll line without event: %s", sc.Text())
+			}
+			got = append(got, m)
+		}
+		return got
+	}
+	if got := pollLines("since=all"); len(got) != 3 {
+		t.Fatalf("poll since=all: %d messages", len(got))
+	}
+	if got := pollLines("since=" + ids[0]); len(got) != 2 || got[0].Message != "two" {
+		t.Fatalf("poll since=<id>: %+v", got)
+	}
+	if got := pollLines("since=" + ids[2]); len(got) != 0 {
+		t.Fatalf("poll since=<last id>: %+v", got)
+	}
+	if got := pollLines("since=10m"); len(got) != 3 {
+		t.Fatalf("poll since=10m: %d messages", len(got))
+	}
+
 	// Reserved topics are refused.
 	for _, reserved := range []string{"/api", "/status"} {
 		r, _ := http.NewRequest("POST", ts.URL+reserved, strings.NewReader("x"))