feat(ocr-pipeline): add row detection step with horizontal gap analysis

Add Step 4 (row detection) between column detection and word recognition.
Uses horizontal projection profiles + whitespace gaps (same method as columns).
Includes header/footer classification via gap-size heuristics.

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

klausur-service/backend/migrations/003_add_row_result.sql

@@ -0,0 +1,4 @@
+-- Migration 003: Add row_result column for row geometry detection
+-- Stores detected row geometries including header/footer classification
+
+ALTER TABLE ocr_pipeline_sessions ADD COLUMN IF NOT EXISTS row_result JSONB;