fix: flush session when removing batch row

otherwise if caller doesn't flush, could become confused.  e.g. for
some reason `batch.row_count` did not appear updated until flush

src/wuttjamaican/batch.py

@@ -309,6 +309,7 @@ class BatchHandler(GenericHandler):
         should (usually) override :meth:`remove_row()` etc.
         """
         batch = row.batch
+        session = self.app.get_session(batch)
 
         self.remove_row(row)
 
@@ -316,6 +317,7 @@ class BatchHandler(GenericHandler):
             batch.row_count -= 1
 
         self.refresh_batch_status(batch)
+        session.flush()
 
     def remove_row(self, row):
         """