Imports Optional type hint

Imports the `Optional` type hint from the `typing` module into `crud.py` and `models.py`.

This provides more explicit type annotations where values can be `None`.

crud.py

@@ -1,4 +1,5 @@
 from datetime import datetime, timedelta, timezone
+from typing import Optional
 
 from lnbits.db import Database
 from lnbits.helpers import urlsafe_short_hash

models.py

@@ -1,4 +1,5 @@
 from datetime import datetime
+from typing import Optional
 
 from fastapi import Query
 from pydantic import BaseModel, EmailStr, root_validator