fix(sse): buffer session events for late subscribers + clear on close

The Pi process produces events immediately after session creation, but
the browser's SSE connection may not be open yet (React re-render delay,
navigation). The hub discarded events with no subscribers, so the user
saw a blank session.

- Ring-buffer up to 200 events per session; replay on subscribe
- hub.clear(id) on POST /sessions/:id/close frees memory

backend/src/routes/sessions.ts

@@ -62,7 +62,9 @@ export function sessionRoutes(
     return reply.code(200).send({ id });
   });
   app.post("/sessions/:id/close", async (req) => {
-    d.mgr.teardown((req.params as any).id);
+    const id = (req.params as { id: string }).id;
+    d.mgr.teardown(id);
+    d.hub.clear(id);
     return { closed: true };
   });
   app.get("/sessions/:id/events", (req, reply) => {