feat: add Woerterbuch category + column add/delete in grid editor

- New document category "Woerterbuch" (frontend type + backend validation)
- Column delete: hover column header → red "x" button (with confirmation)
- Column add: hover column header → "+" button inserts after that column
- Both operations support undo/redo, update cell IDs and summary
- Available in both GridEditor and StepGridReview (Kombi last step)

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

klausur-service/backend/ocr_pipeline_common.py

@@ -146,7 +146,7 @@ class DewarpGroundTruthRequest(BaseModel):
 
 
 VALID_DOCUMENT_CATEGORIES = {
-    'vokabelseite', 'buchseite', 'arbeitsblatt', 'klausurseite',
+    'vokabelseite', 'woerterbuch', 'buchseite', 'arbeitsblatt', 'klausurseite',
     'mathearbeit', 'statistik', 'zeitung', 'formular', 'handschrift', 'sonstiges',
 }