fix: page property type

[joscha]

This actually results in page properties not being deserialized correctly at the moment - each property in Page.properties is a dict instead of a BaseModel subtype.

Current workaround is:

page_property_adapter = TypeAdapter(PageProperty)
...
for prop_raw in page.properties
    if prop_raw is None:
        continue
    prop: PageProperty = page_property_adapter.validate_python(prop_raw)