Fix IPA marker split: walk back max 3 chars for onset cluster

The walk-back was going 4 chars, eating the last letter of the
headword: "schoolbag" → "schoolba". Limiting to 3 gives correct
split: "schoolbag" + "[sku:lbæg]".

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

klausur-service/backend/cv_ocr_engines.py

@@ -1073,7 +1073,7 @@ def _insert_missing_ipa(text: str, pronunciation: str = 'british') -> str:
             if first_marker >= 3:
                 split = first_marker
                 while (split > 0
-                       and split > first_marker - 4
+                       and split > first_marker - 3
                        and w[split - 1].isalpha()
                        and w[split - 1].islower()):
                     split -= 1