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/main.py

@@ -43,6 +43,7 @@ except ImportError:
     trocr_router = None
 from vocab_worksheet_api import router as vocab_router, set_db_pool as set_vocab_db_pool, _init_vocab_table, _load_all_sessions, DATABASE_URL as VOCAB_DATABASE_URL
 from ocr_pipeline_api import router as ocr_pipeline_router, _cache as ocr_pipeline_cache
+from grid_editor_api import router as grid_editor_router
 from orientation_crop_api import router as orientation_crop_router, set_cache_ref as set_orientation_crop_cache
 from ocr_pipeline_session_store import init_ocr_pipeline_tables
 try:
@@ -178,6 +179,7 @@ if trocr_router:
     app.include_router(trocr_router)      # TrOCR Handwriting OCR
 app.include_router(vocab_router)      # Vocabulary Worksheet Generator
 app.include_router(ocr_pipeline_router)  # OCR Pipeline (step-by-step)
+app.include_router(grid_editor_router)   # Grid Editor (Excel-like)
 set_orientation_crop_cache(ocr_pipeline_cache)
 app.include_router(orientation_crop_router)  # OCR Pipeline: Orientation + Crop
 if htr_router: