enhance rag retrieval + summary

backend/src/main/java/com/aiteacher/topic/SavedSummaryItem.java

@@ -0,0 +1,7 @@
+package com.aiteacher.topic;
+
+import java.time.Instant;
+import java.util.UUID;
+
+public record SavedSummaryItem(UUID id, int summaryNumber, Instant generatedAt) {
+}