improve topics and chat source display

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

@@ -19,9 +19,11 @@ public record TopicSummaryResponse(
         String bookId,
         String bookTitle,
         Integer page,
+        String chunkText,
         String figureId,
         String label,
         String caption,
+        String figureType,
         String imageUrl
     ) {
     }

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

@@ -188,7 +188,7 @@ public class TopicSummaryService {
             String bookId = book != null ? book.getId().toString() : null;
             sources.add(new TopicSummaryResponse.SourceReference(
                 "TEXT", "S" + (i + 1), bookId, title, s.getPageStart(),
-                null, null, null, null));
+                truncate(s.getFullText(), 500), null, null, null, null, null));
         }
 
         for (int i = 0; i < figures.size(); i++) {
@@ -203,7 +203,8 @@ public class TopicSummaryService {
             String imageUrl = "/api/v1/figures/" + f.getBookId() + "/" + filename;
             sources.add(new TopicSummaryResponse.SourceReference(
                 "FIGURE", "F" + (i + 1), bookId, title, f.getPage(),
-                f.getId(), f.getLabel(), f.getCaption(), imageUrl));
+                null, f.getId(), f.getLabel(), f.getCaption(),
+                f.getFigureType().name(), imageUrl));
         }
 
         return sources;
@@ -218,6 +219,11 @@ public class TopicSummaryService {
         }
     }
 
+    private String truncate(String text, int maxChars) {
+        if (text == null) return "";
+        return text.length() <= maxChars ? text : text.substring(0, maxChars) + "…";
+    }
+
     private List<TopicSummaryResponse.SourceReference> deserializeSources(String json) {
         try {
             return objectMapper.readValue(json,