Refine garbled IPA filter: skip only pure-ASCII garbled text, not text with real IPA

"Theme [θˈiːm]" contains real IPA symbols (θ, ˈ) and should NOT be filtered.
Only filter text that has garbled IPA markers (:, ') but no real Unicode IPA chars.

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

klausur-service/backend/grid_editor_api.py

@@ -737,7 +737,9 @@ def _detect_heading_rows_by_single_cell(
             if not text or text.startswith("["):
                 continue
             # Skip garbled IPA without brackets (e.g. "ska:f – ska:vz")
-            if _text_has_garbled_ipa(text):
+            # but NOT text with real IPA symbols (e.g. "Theme [θˈiːm]")
+            _REAL_IPA_CHARS = set("ˈˌəɪɛɒʊʌæɑɔʃʒθðŋ")
+            if _text_has_garbled_ipa(text) and not any(c in _REAL_IPA_CHARS for c in text):
                 continue
             heading_row_indices.append(ri)