fix: deduplicate overlapping OCR words and use per-word Y positions in overlay

Backend: Add spatial overlap check (>=50% horizontal IoU) to Kombi merge
so words at the same position are deduplicated even when OCR text differs.

Frontend: Add yPct/hPct to WordPosition so each word renders at its actual
vertical position instead of all words collapsing to the cell center Y.

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

admin-lehrer/components/ocr-overlay/usePixelWordPositions.ts

@@ -4,6 +4,8 @@ import type { GridCell } from '@/app/(admin)/ai/ocr-overlay/types'
 export interface WordPosition {
   xPct: number
   wPct: number
+  yPct: number
+  hPct: number
   text: string
   fontRatio: number
 }
@@ -192,6 +194,8 @@ export function usePixelWordPositions(
             wordPos.push({
               xPct: cell.bbox_pct.x + (cl.start / cw) * cell.bbox_pct.w,
               wPct: ((cl.end - cl.start + 1) / cw) * cell.bbox_pct.w,
+              yPct: cell.bbox_pct.y,
+              hPct: cell.bbox_pct.h,
               text: groups[gi],
               fontRatio,
             })
@@ -209,6 +213,8 @@ export function usePixelWordPositions(
           wordPos.push({
             xPct: cell.bbox_pct.x + (widest.start / cw) * cell.bbox_pct.w,
             wPct: ((widest.end - widest.start + 1) / cw) * cell.bbox_pct.w,
+            yPct: cell.bbox_pct.y,
+            hPct: cell.bbox_pct.h,
             text: cell.text.trim(),
             fontRatio,
           })