fix: preserve pipe syllable dividers + detect alphabet sidebar columns

1. Pipe divider fix: Changed OCR char-confusion regex so | between
   letters (Ka|me|rad) is NOT converted to I. Only standalone/
   word-boundary pipes are converted (|ch → Ich, | want → I want).

2. Alphabet sidebar detection improvements:
   - _filter_decorative_margin() now considers 2-char words (OCR reads
     "Aa", "Bb" from sidebars), lowered min strip from 8→6
   - _filter_border_strip_words() lowered decorative threshold from 50%→45%
   - New step 4f: grid-level thin-edge-column filter as safety net —
     removes edge columns with <35% fill rate and >60% short text

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

klausur-service/backend/cv_ocr_engines.py

@@ -481,8 +481,9 @@ _CHAR_CONFUSION_RULES = [
     (re.compile(r'\b1([a-z])'), r'I\1'),           # 1ch → Ich, 1want → Iwant
     # Standalone "1" → "I" (English pronoun), but NOT "1." or "1," (list number)
     (re.compile(r'(?<!\d)\b1\b(?![\d.,])'), 'I'),  # "1 want" → "I want"
-    # "|" → "I", but NOT "|." or "|," (those are "1." list prefixes → spell-checker handles them)
-    (re.compile(r'(?<!\|)\|(?!\||[.,])'), 'I'),    # |ch → Ich, | want → I want
+    # "|" → "I", but NOT when embedded between letters (syllable divider: Ka|me|rad)
+    # and NOT "|." or "|," (those are "1." list prefixes → spell-checker handles them)
+    (re.compile(r'(?<![a-zA-ZäöüÄÖÜß])\|(?!\||[.,])'), 'I'),  # |ch → Ich, | want → I want
 ]
 
 # Cross-language indicators: if DE has these, EN "1" is almost certainly "I"