fix: improve import, evidence indexing and material-scoped grading

Preserve document revisions and evidence, harden indexing and export behavior, add explicit material selection and direct-folder import, validate model JSON schemas with safe error messages, and retain student selection while loading scoring data.

backend/wisegrade/actions.py

@@ -290,6 +290,8 @@ def confirm(db, conversation: Conversation, action_id: int):
         db.commit()
         return result
     except Exception as exc:
+        db.rollback()
+        action = db.get(PendingAction, action_id)
         action.status, action.result = "failed", {"error": str(exc)[:300]}
         db.commit()
         raise