mqtt test: use playback start as reliable trigger, drop auto-prefix

two fixes based on actual behavior of the jellyfin webhook plugin:

- 'Webhook Url' setup value no longer re-serialized with mqtt://. show
  the user's broker url verbatim so whatever protocol they use (ws://,
  http://, etc.) survives the round trip
- dropped the server-side 'trigger a jellyfin rescan during the test'
  machinery. a refresh that doesn't mutate metadata won't fire Item
  Added, so relying on it produced false negatives. now we just wait
  for any message on the topic; ui instructs the user to hit play on a
  movie in jellyfin while the test runs — playback start is a
  deterministic trigger, unlike library events
- setup panel now lists Notification Types as 'Item Added, Playback
  Start'. playback start is for the test only; the production handler
  still filters events down to item added / updated

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "netfelix-audio-fix",
-	"version": "2026.04.14.6",
+	"version": "2026.04.14.7",
 	"scripts": {
 		"dev:server": "NODE_ENV=development bun --hot server/index.tsx",
 		"dev:client": "vite",