Fix word-gap merge: add missing pronouns to stop words, reduce threshold

- Add du/dich/dir/mich/mir/uns/euch/ihm/ihn to _STOP_WORDS to prevent
  false merges like "du" + "zerlegst" → "duzerlegst"
- Reduce max_short threshold from 6 to 5 to prevent merging multi-word
  phrases like "ziehen lassen" → "ziehenlassen"

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

klausur-service/backend/cv_syllable_detect.py

@@ -34,7 +34,8 @@ _STOP_WORDS = frozenset([
     'der', 'die', 'das', 'dem', 'den', 'des',
     'ein', 'eine', 'einem', 'einen', 'einer',
     # Pronouns
-    'er', 'es', 'sie', 'wir', 'ihr', 'ich', 'man', 'sich',
+    'du', 'er', 'es', 'sie', 'wir', 'ihr', 'ich', 'man', 'sich',
+    'dich', 'dir', 'mich', 'mir', 'uns', 'euch', 'ihm', 'ihn',
     # Prepositions
     'mit', 'von', 'zu', 'für', 'auf', 'in', 'an', 'um', 'am', 'im',
     'aus', 'bei', 'nach', 'vor', 'bis', 'durch', 'über', 'unter',
@@ -146,7 +147,7 @@ def merge_word_gaps_in_zones(zones_data: List[Dict], session_id: str) -> int:
     producing text like "zerknit tert" instead of "zerknittert".  This
     function tries to merge adjacent fragments in every content cell.
 
-    More permissive than ``_try_merge_pipe_gaps`` (threshold 6 instead of 3)
+    More permissive than ``_try_merge_pipe_gaps`` (threshold 5 instead of 3)
     but still guarded by pyphen dictionary lookup and stop-word exclusion.
 
     Returns the number of cells modified.
@@ -186,8 +187,9 @@ def merge_word_gaps_in_zones(zones_data: List[Dict], session_id: str) -> int:
 def _try_merge_word_gaps(text: str, hyph_de) -> str:
     """Merge OCR word fragments with relaxed threshold (max_short=6).
 
-    Similar to ``_try_merge_pipe_gaps`` but allows longer fragments to be
-    merged.  Still requires pyphen to recognize the merged word.
+    Similar to ``_try_merge_pipe_gaps`` but allows slightly longer fragments
+    (max_short=5 instead of 3).  Still requires pyphen to recognize the
+    merged word.
     """
     parts = text.split(' ')
     if len(parts) < 2:
@@ -207,7 +209,7 @@ def _try_merge_word_gaps(text: str, hyph_de) -> str:
             and prev_alpha and curr_alpha
             and prev_alpha.lower() not in _STOP_WORDS
             and curr_alpha.lower() not in _STOP_WORDS
-            and min(len(prev_alpha), len(curr_alpha)) <= 6
+            and min(len(prev_alpha), len(curr_alpha)) <= 5
             and len(prev_alpha) + len(curr_alpha) >= 4
         )