Fix sorting by referenced_by_count with unmapped_type handling

- Added unmapped_type parameter to referenced_by_count sort
- This handles documents that don't have the field yet
- Updated ingest.py to include reference fields when indexing:
  * internal_references
  * internal_references_count
  * referenced_by
  * referenced_by_count
- Updated index mapping to include reference fields
- Documents without the field will sort as 0 (appear last)

Fixes BadRequestError: No mapping found for [referenced_by_count]

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

search_app.py

@@ -400,7 +400,7 @@ def build_query_payload(
         elif sort == "older":
             body["sort"] = [{"date": {"order": "asc"}}]
         elif sort == "referenced":
-            body["sort"] = [{"referenced_by_count": {"order": "desc"}}]
+            body["sort"] = [{"referenced_by_count": {"order": "desc", "unmapped_type": "long"}}]
         return body
 
     if query:
@@ -505,7 +505,7 @@ def build_query_payload(
     elif sort == "older":
         body["sort"] = [{"date": {"order": "asc"}}]
     elif sort == "referenced":
-        body["sort"] = [{"referenced_by_count": {"order": "desc"}}]
+        body["sort"] = [{"referenced_by_count": {"order": "desc", "unmapped_type": "long"}}]
     return body