add updated_at to BookVersion

app/models/book_version.py

@@ -1,3 +1,5 @@
+from datetime import datetime
+
 from app import db
 from app.models.utils import BaseModel
 
@@ -9,6 +11,7 @@ class BookVersion(BaseModel):
     id = db.Column(db.Integer, primary_key=True)
     semver = db.Column(db.String(1024), unique=False, nullable=False)
     exported = db.Column(db.Boolean, default=False)
+    updated_at = db.Column(db.DateTime, default=datetime.now)
 
     # Foreign keys
     derivative_id = db.Column(db.Integer, db.ForeignKey("book_versions.id"))