fix: suppress text fragments in graphic detection

- Raise min_area from 30 to 200 (text fragments are small)
- Raise word_pad from 3 to 10px (OCR bboxes are tight)
- Reduce morph close kernel from 5x5 to 3x3 (avoid reconnecting text)
- Tighten arrow detection: min 20px, circularity<0.35, >=2 defects
- Add 'noise' category for too-small elements, filter them out
- Raise min dimension from 4 to 8px
- Add debug logging for word count and exclusion coverage
- Raise max_area_ratio to 0.25 (allow larger illustrations)

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

klausur-service/backend/ocr_pipeline_api.py

@@ -1236,6 +1236,10 @@ async def detect_structure(session_id: str):
         for cell in word_result["cells"]:
             for wb in (cell.get("word_boxes") or []):
                 words.append(wb)
+    logger.info("detect-structure: word_result present=%s, cells=%d, word_boxes extracted=%d",
+                word_result is not None,
+                len(word_result.get("cells", [])) if word_result else 0,
+                len(words))
     # If no words yet, use image dimensions with small margin
     if words:
         content_x = max(0, min(int(wb["left"]) for wb in words))