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/app/(admin)/ai/ocr-pipeline/types.ts

@@ -219,7 +219,7 @@ export interface StructureGraphic {
   w: number
   h: number
   area: number
-  shape: string   // arrow, circle, line, exclamation, dot, icon, illustration
+  shape: string   // image, illustration
   color_name: string
   color_hex: string
   confidence: number
@@ -235,6 +235,7 @@ export interface StructureResult {
   color_pixel_counts: Record<string, number>
   has_words: boolean
   word_count: number
+  border_ghosts_removed?: number
   duration_seconds: number
 }