fix: improve PP-StructureV3 structure preservation for complex diagrams

- Fix parsing_res_list field mapping (block_label, block_content, block_bbox)
- Add fine-grained PP-StructureV3 configuration parameters
- Lower detection thresholds (0.5→0.2) for more sensitive element detection
- Use 'small' merge mode instead of default to minimize bbox merging
- Add layout_nms, unclip_ratio, text_det thresholds for better control
- Result: Doubled element detection from 6 to 12 elements on complex diagrams

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/app/services/ocr_service.py

@@ -359,8 +359,16 @@ class OCRService:
                 use_formula = settings.enable_formula_recognition
                 use_table = settings.enable_table_recognition
                 layout_threshold = settings.layout_detection_threshold
+                layout_nms = settings.layout_nms_threshold
+                layout_merge = settings.layout_merge_mode
+                layout_unclip = settings.layout_unclip_ratio
+                text_thresh = settings.text_det_thresh
+                text_box_thresh = settings.text_det_box_thresh
+                text_unclip = settings.text_det_unclip_ratio
 
                 logger.info(f"PP-StructureV3 config: table={use_table}, formula={use_formula}, chart={use_chart}")
+                logger.info(f"Layout config: threshold={layout_threshold}, nms={layout_nms}, merge={layout_merge}, unclip={layout_unclip}")
+                logger.info(f"Text detection: thresh={text_thresh}, box_thresh={text_box_thresh}, unclip={text_unclip}")
 
                 self.structure_engine = PPStructureV3(
                     use_doc_orientation_classify=False,
@@ -368,8 +376,14 @@ class OCRService:
                     use_textline_orientation=False,
                     use_table_recognition=use_table,
                     use_formula_recognition=use_formula,
-                    use_chart_recognition=use_chart,  # Disabled by default to save ~500MB VRAM
+                    use_chart_recognition=use_chart,
                     layout_threshold=layout_threshold,
+                    layout_nms=layout_nms,
+                    layout_unclip_ratio=layout_unclip,
+                    layout_merge_bboxes_mode=layout_merge,  # Use 'small' to minimize merging
+                    text_det_thresh=text_thresh,
+                    text_det_box_thresh=text_box_thresh,
+                    text_det_unclip_ratio=text_unclip,
                 )
 
                 # Track model loading for cache management