Add mentions to render_message for better BM25 recall

When a question references a user by name/id, sparse search now finds
chunks where that user was mentioned even if not the sender.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index/main.py

@@ -101,6 +101,9 @@ def render_message(message: Message) -> str:
                 else:
                     parts_list.append(part_text)
 
+    if message.mentions:
+        parts_list.append(" ".join(message.mentions))
+
     if message.file_snippets:
         parts_list.append(f"[Файл]: {message.file_snippets}")