feat: add Excel-like grid editor for OCR overlay (Kombi mode step 6)

Backend: new grid_editor_api.py with build-grid endpoint that detects
bordered boxes, splits page into zones, clusters columns/rows per zone
from Kombi word positions. New DB column grid_editor_result JSONB.

Frontend: GridEditor component with editable HTML tables per zone,
column bold toggle, header row toggle, undo/redo, keyboard navigation
(Tab/Enter/Arrow), image overlay verification, and save/load.

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

klausur-service/backend/migrations/007_add_grid_editor_result.sql

@@ -0,0 +1,4 @@
+-- Migration 007: Add grid_editor_result column for Excel-like grid editor
+-- Stores structured grid with zones, columns, rows, cells, and formatting
+
+ALTER TABLE ocr_pipeline_sessions ADD COLUMN IF NOT EXISTS grid_editor_result JSONB;