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/ocr_pipeline_session_store.py

@@ -74,7 +74,8 @@ async def init_ocr_pipeline_tables():
             ADD COLUMN IF NOT EXISTS orientation_result JSONB,
             ADD COLUMN IF NOT EXISTS crop_result JSONB,
             ADD COLUMN IF NOT EXISTS parent_session_id UUID REFERENCES ocr_pipeline_sessions(id) ON DELETE CASCADE,
-            ADD COLUMN IF NOT EXISTS box_index INT
+            ADD COLUMN IF NOT EXISTS box_index INT,
+            ADD COLUMN IF NOT EXISTS grid_editor_result JSONB
         """)
 
 
@@ -110,6 +111,7 @@ async def create_session_db(
                       word_result, ground_truth, auto_shear_degrees,
                       doc_type, doc_type_result,
                       document_category, pipeline_log,
+                      grid_editor_result,
                       parent_session_id, box_index,
                       created_at, updated_at
         """, uuid.UUID(session_id), name, filename, original_png,
@@ -129,6 +131,7 @@ async def get_session_db(session_id: str) -> Optional[Dict[str, Any]]:
                    word_result, ground_truth, auto_shear_degrees,
                    doc_type, doc_type_result,
                    document_category, pipeline_log,
+                   grid_editor_result,
                    parent_session_id, box_index,
                    created_at, updated_at
             FROM ocr_pipeline_sessions WHERE id = $1
@@ -180,10 +183,11 @@ async def update_session_db(session_id: str, **kwargs) -> Optional[Dict[str, Any
         'word_result', 'ground_truth', 'auto_shear_degrees',
         'doc_type', 'doc_type_result',
         'document_category', 'pipeline_log',
+        'grid_editor_result',
         'parent_session_id', 'box_index',
     }
 
-    jsonb_fields = {'orientation_result', 'crop_result', 'deskew_result', 'dewarp_result', 'column_result', 'row_result', 'word_result', 'ground_truth', 'handwriting_removal_meta', 'doc_type_result', 'pipeline_log'}
+    jsonb_fields = {'orientation_result', 'crop_result', 'deskew_result', 'dewarp_result', 'column_result', 'row_result', 'word_result', 'ground_truth', 'handwriting_removal_meta', 'doc_type_result', 'pipeline_log', 'grid_editor_result'}
 
     for key, value in kwargs.items():
         if key in allowed_fields:
@@ -212,6 +216,7 @@ async def update_session_db(session_id: str, **kwargs) -> Optional[Dict[str, Any
                       word_result, ground_truth, auto_shear_degrees,
                       doc_type, doc_type_result,
                       document_category, pipeline_log,
+                      grid_editor_result,
                       parent_session_id, box_index,
                       created_at, updated_at
         """, *values)
@@ -308,7 +313,7 @@ def _row_to_dict(row: asyncpg.Record) -> Dict[str, Any]:
             result[key] = result[key].isoformat()
 
     # JSONB → parsed (asyncpg returns str for JSONB)
-    for key in ['orientation_result', 'crop_result', 'deskew_result', 'dewarp_result', 'column_result', 'row_result', 'word_result', 'ground_truth', 'doc_type_result', 'pipeline_log']:
+    for key in ['orientation_result', 'crop_result', 'deskew_result', 'dewarp_result', 'column_result', 'row_result', 'word_result', 'ground_truth', 'doc_type_result', 'pipeline_log', 'grid_editor_result']:
         if key in result and result[key] is not None:
             if isinstance(result[key], str):
                 result[key] = json.loads(result[key])