fix: harden resume and SSE replay

backend/src/bridge/session-bridge.ts

@@ -13,7 +13,7 @@ export type ClientEvent =
   | { type: "activity_delta"; text: string }
   | { type: "activity_event"; activity: ToolActivity }
   | { type: "info"; [k: string]: any }
-  | { type: "system_event"; [k: string]: any };
+  | { type: "system_event"; event: string };
 
 export type TurnState = "idle" | "running" | "waiting" | "failed";
 
@@ -61,7 +61,9 @@ export class SessionBridge {
         this.emitToolActivity(m, "end");
       // Tool updates and raw payloads remain intentionally dropped.
       } else if (m.type === "system_event") {
-        this.fan(m as ClientEvent);
+        if (typeof m.event === "string" && m.event.trim() !== "") {
+          this.fan({ type: "system_event", event: m.event });
+        }
       } else if (m.type === "agent_end") {
         if (this.state !== "failed" && !this.pending) this.state = "idle";
         this.fan({ type: "system_event", event: "agent_end" });