Add backend archive listing endpoint

api.py

@@ -466,6 +466,19 @@ def get_session(session_id: str, request: Request):
     return normalize_session_payload(session, request)
 
 
+@app.get("/api/archive")
+def get_archive(request: Request):
+    sessions = []
+    for session_file in list_session_files():
+        try:
+            with open(session_file, "r", encoding="utf-8") as f:
+                session = json.load(f)
+            sessions.append(normalize_session_payload(session, request))
+        except Exception:
+            continue
+    return {"sessions": sessions}
+
+
 @app.delete("/api/sessions/{session_id}")
 def delete_session(session_id: str):
     session_dir = find_session_path(session_id)