feat(ocr-pipeline): line grouping fix + RapidOCR integration

Fix A: Use _group_words_into_lines() with adaptive Y-tolerance to
correctly order words in multi-line cells (fixes word reordering bug).

RapidOCR: Add as alternative OCR engine (PaddleOCR models on ONNX
Runtime, native ARM64). Engine selectable via dropdown in UI or
?engine= query param. Auto mode prefers RapidOCR when available.

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

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

@@ -143,6 +143,7 @@ export interface WordResult {
   image_width: number
   image_height: number
   duration_seconds: number
+  ocr_engine?: string
   summary: {
     total_entries: number
     with_english: number