Fix 4 Grid Editor bugs: syllable modes, heading detection, word gaps

1. Syllable "Original" (auto) mode: only normalize cells that already
   have | from OCR — don't add new syllable marks via pyphen to words
   without printed dividers on the original scan.

2. Syllable "Aus" (none) mode: strip residual | chars from OCR text
   so cells display clean (e.g. "Zel|le" → "Zelle").

3. Heading detection: add text length guard in single-cell heuristic —
   words > 4 alpha chars starting lowercase (like "zentral") are regular
   vocabulary, not section headings.

4. Word-gap merge: new merge_word_gaps_in_zones() step with relaxed
   threshold (6 chars) fixes OCR splits like "zerknit tert" → "zerknittert".

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

klausur-service/backend/grid_editor_api.py

@@ -1593,6 +1593,13 @@ async def _build_grid_core(
     except Exception as e:
         logger.warning("Dictionary detection failed: %s", e)
 
+    # --- Word-gap merge: fix OCR splits like "zerknit tert" → "zerknittert" ---
+    try:
+        from cv_syllable_detect import merge_word_gaps_in_zones
+        merge_word_gaps_in_zones(zones_data, session_id)
+    except Exception as e:
+        logger.warning("Word-gap merge failed: %s", e)
+
     # --- Syllable divider insertion for dictionary pages ---
     # syllable_mode: "auto" = only when original has pipe dividers (1% threshold),
     #   "all" = force on all content words, "en" = English column only,
@@ -1626,6 +1633,15 @@ async def _build_grid_core(
             except Exception as e:
                 logger.warning("Syllable insertion failed: %s", e)
 
+    # When syllable mode is "none", strip any residual | from OCR so
+    # that the displayed text is clean (e.g. "Zel|le" → "Zelle").
+    if syllable_mode == "none":
+        for z in zones_data:
+            for cell in z.get("cells", []):
+                t = cell.get("text", "")
+                if "|" in t:
+                    cell["text"] = t.replace("|", "")
+
     # Clean up internal flags before returning
     for z in zones_data:
         for cell in z.get("cells", []):