feat(ocr-pipeline): add Step 5 word recognition (grid from columns × rows)

Backend: build_word_grid() intersects column regions with content rows,
OCRs each cell with language-specific Tesseract, and returns vocabulary
entries with percent-based bounding boxes. New endpoints: POST /words,
GET /image/words-overlay, ground-truth save/retrieve for words.
Frontend: StepWordRecognition with overview + step-through labeling modes,
goToStep callback for row correction feedback loop.
MkDocs: OCR Pipeline documentation added.

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

klausur-service/backend/migrations/004_add_word_result.sql

@@ -0,0 +1,4 @@
+-- Migration 004: Add word_result column for OCR Pipeline Step 5
+-- Stores the word recognition grid result (entries with english/german/example + bboxes)
+
+ALTER TABLE ocr_pipeline_sessions ADD COLUMN IF NOT EXISTS word_result JSONB;