fix: make session resume atomic across restarts

backend/src/sse/sse-hub.ts

@@ -91,4 +91,10 @@ export class SseHub {
     this.buffers.delete(sessionId);
     this.subs.delete(sessionId);
   }
+
+  /** Permanently discard transport state for a deleted session id. */
+  forget(sessionId: string): void {
+    this.clear(sessionId);
+    this.lastIds.delete(sessionId);
+  }
 }