enhance rag retrieval + summary

backend/src/main/java/com/aiteacher/retrieval/ExpandedQuery.java

@@ -0,0 +1,7 @@
+package com.aiteacher.retrieval;
+
+/**
+ * Value object holding the original user query alongside its clinically
+ * rewritten variant used for vector-store retrieval.
+ */
+public record ExpandedQuery(String original, String rewritten) {}