Harden live camera viewer sync rendering

tests/e2e/helpers/mockApp.ts

@@ -754,13 +754,17 @@ async function handleMedia(route: Route, state: MockAppState) {
 
   if (path === `/media/sessions/${state.mediaSession.id}`) {
     if (state.mediaSession.purpose === "relay") {
-      state.mediaSession.previewStatus = "ready";
+      state.mediaSession.previewStatus = state.mediaSession.playback.previewUrl
+        ? "ready"
+        : "processing";
       state.mediaSession.previewUpdatedAt = nowIso();
       state.mediaSession.playback = {
-        ready: true,
-        webmUrl: "/media/assets/sessions/session-e2e/preview.webm",
-        webmSize: 1_800_000,
-        previewUrl: "/media/assets/sessions/session-e2e/preview.webm",
+        ready: Boolean(state.mediaSession.playback.previewUrl),
+        webmUrl:
+          state.mediaSession.playback.webmUrl ??
+          "/media/assets/sessions/session-e2e/preview.webm",
+        webmSize: state.mediaSession.playback.webmSize ?? 1_800_000,
+        previewUrl: state.mediaSession.playback.previewUrl,
       };
     } else {
       state.mediaSession.status = "archived";