Tutor/answers: bare subscripts (S_n, a_1) become Unicode; migration 0022 re-cleans DB text

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

ai_tutor/tests_plain_history.py

@@ -23,3 +23,11 @@ class HistoryNeverShowsLatexTests(TestCase):
             self.assertNotIn('^', t)
             self.assertNotIn('$', t)
         self.assertIn('x²', texts[0])
+
+
+class ToPlainSubscriptTests(TestCase):
+    def test_bare_subscripts_are_unicode(self):
+        from rag.services.mathtext import to_plain
+        out = to_plain('S_n = n/2 × (a_1 + a_n)')
+        self.assertNotIn('_', out)
+        self.assertIn('Sₙ', out)