Preserve grammar annotations (pl), (no pl) and skip articles in IPA

Two fixes:
1. Add pl, sg, no, also, ae, be etc. to _GRAMMAR_BRACKET_WORDS so
   annotations like (pl) and (no pl) are not replaced with IPA.
2. Skip articles (the, a, an) in fix_ipa_continuation_cell — they
   never get IPA in vocabulary books.

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

klausur-service/backend/tests/test_grid_editor_api.py

@@ -485,13 +485,14 @@ class TestGarbledIpaDetection:
         assert _text_has_garbled_ipa("equipment") is False
 
     def test_fix_continuation_united_kingdom(self):
-        """IPA continuation for 'the United Kingdom' → proper IPA."""
+        """IPA continuation for 'the United Kingdom' → IPA without 'the'."""
         fixed = fix_ipa_continuation_cell(
             "[n, nn]", "the United Kingdom", pronunciation="british",
         )
         # Should contain proper IPA, not the garbled text
         assert fixed != "[n, nn]"
         assert "kˈɪŋdəm" in fixed  # Kingdom IPA
+        assert "ðə" not in fixed    # "the" must NOT get IPA
 
     def test_fix_continuation_equipment(self):
         """IPA continuation for 'equipment' → proper IPA."""