feat: add border ghost filter + graphic detection tests + structure overlay

- Add _filter_border_ghost_words() to remove OCR artefacts from box borders
  (vertical + horizontal edge detection, column cleanup, re-indexing)
- Add 20 tests for border ghost filter (basic filtering + column cleanup)
- Add 24 tests for cv_graphic_detect (color detection, word overlap, boxes)
- Clean up cv_graphic_detect.py logging (per-candidate → DEBUG)
- Add structure overlay layer to StepReconstruction (boxes + graphics toggle)
- Show border_ghosts_removed badge in StepStructureDetection
- Update MkDocs with structure detection documentation

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

admin-lehrer/components/ocr-pipeline/StepStructureDetection.tsx

@@ -165,6 +165,11 @@ export function StepStructureDetection({ sessionId, onNext }: StepStructureDetec
                 {result.word_count} Woerter
               </span>
             )}
+            {(result.border_ghosts_removed ?? 0) > 0 && (
+              <span className="inline-flex items-center gap-1.5 px-3 py-1 rounded-full bg-red-50 dark:bg-red-900/20 text-red-700 dark:text-red-400 text-xs font-medium">
+                {result.border_ghosts_removed} Rahmenlinien entfernt
+              </span>
+            )}
             <span className="text-gray-400 text-xs ml-auto">
               {result.image_width}x{result.image_height}px | {result.duration_seconds}s
             </span>