Support flush() for Tantivy

quickpeep_index/src/backend/tantivy.rs

@@ -105,6 +105,9 @@ impl Backend for TantivyBackend {
     }
 
     fn flush(&mut self) -> anyhow::Result<()> {
-        todo!()
+        if let Some(index_writer) = self.index_writer.as_mut() {
+            index_writer.commit()?;
+        }
+        Ok(())
     }
 }