fix(telemetry): idle-based session rotation; faster sse keepalive

- session id rotates after 30 idle minutes so installed pwas produce
  visit-sized sessions instead of one eternal id
- sse ping every 8s, under the idle timeout observed killing streams

src/app/routes/api/events.$.ts

@@ -39,9 +39,10 @@ export const Route = createFileRoute("/api/events/$")({
               serverEvents.on(type, handleEvent);
             }
 
+            // Under the shortest idle timeout observed on proxies/PWA paths.
             const pingInterval = setInterval(() => {
               send({ type: "ping", timestamp: Date.now() });
-            }, 15000);
+            }, 8000);
 
             cleanup = () => {
               if (cleanedUp) return;

src/lib/telemetry/client/index.ts

@@ -17,19 +17,37 @@ const recentErrors = new Map<string, number>();
 
 const isEnabled = (): boolean => typeof window !== "undefined";
 
-const getSessionId = (): string => {
+// A session ends after 30 idle minutes, not when the tab closes — installed
+// PWAs keep sessionStorage alive for days.
+const SESSION_IDLE_MS = 30 * 60 * 1000;
+const SESSION_KEY = "flxn-tsid";
+
+const readSession = (): { id: string; t: number } | null => {
   try {
-    let id = sessionStorage.getItem("flxn-tsid");
-    if (!id) {
-      id = crypto.randomUUID();
-      sessionStorage.setItem("flxn-tsid", id);
+    const raw = sessionStorage.getItem(SESSION_KEY);
+    if (!raw) return null;
+    const parsed = JSON.parse(raw);
+    if (typeof parsed?.id === "string" && typeof parsed?.t === "number") {
+      return parsed;
     }
-    return id;
+    return null;
   } catch {
-    return "anon-session";
+    return null;
   }
 };
 
+const touchSession = () => {
+  try {
+    const now = Date.now();
+    const current = readSession();
+    const id =
+      current && now - current.t <= SESSION_IDLE_MS ? current.id : crypto.randomUUID();
+    sessionStorage.setItem(SESSION_KEY, JSON.stringify({ id, t: now }));
+  } catch {}
+};
+
+const getSessionId = (): string => readSession()?.id ?? "anon-session";
+
 const location = (): { path: string; routeId?: string } => ({
   path: window.location.pathname,
   routeId: lastRouteId,
@@ -64,6 +82,7 @@ const flush = (useBeacon = false) => {
 
 const enqueue = (event: QueuedEvent) => {
   if (!isEnabled()) return;
+  touchSession();
   queue.push(event);
   if (queue.length >= FLUSH_AT) {
     flush();