feat: add table detection options and scan artifact removal

- Add TableDetectionSelector component for wired/wireless/region detection
- Add CV-based table line detector module (disabled due to poor performance)
- Add scan artifact removal preprocessing step (removes faint horizontal lines)
- Add PreprocessingConfig schema with remove_scan_artifacts option
- Update frontend PreprocessingSettings with scan artifact toggle
- Integrate table detection config into ProcessingPage
- Archive extract-table-cell-boxes proposal

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

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

backend/app/services/gap_filling_service.py

@@ -212,7 +212,8 @@ class GapFillingService:
     def _is_region_covered(
         self,
         region: TextRegion,
-        pp_structure_elements: List[DocumentElement]
+        pp_structure_elements: List[DocumentElement],
+        skip_table_coverage: bool = True
     ) -> bool:
         """
         Check if a raw OCR region is covered by any PP-StructureV3 element.
@@ -220,6 +221,9 @@ class GapFillingService:
         Args:
             region: Raw OCR text region
             pp_structure_elements: List of PP-StructureV3 elements
+            skip_table_coverage: If True, don't consider TABLE elements as covering
+                                 (allows raw OCR text inside tables to pass through
+                                 for layered rendering)
 
         Returns:
             True if the region is covered
@@ -228,6 +232,12 @@ class GapFillingService:
         region_bbox = region.normalized_bbox
 
         for element in pp_structure_elements:
+            # Skip TABLE elements when checking coverage
+            # This allows raw OCR text inside tables to be preserved
+            # PDF generator will render: table borders + raw text positions
+            if skip_table_coverage and element.type == ElementType.TABLE:
+                continue
+
             elem_bbox = (
                 element.bbox.x0, element.bbox.y0,
                 element.bbox.x1, element.bbox.y1