Where is Scaffolding data stored?

Answer

Scaffolding data is stored in the table OS_PROPERTYENTRY with entity_id as a foreign key to CONTENT.contentid.

The history/versioning of Confluence leads to many entries with the same OS_PROPERTYENTRY.entity_id.
The latest/current version is the entry with OS_PROPERTYENTRY.entity_key = "~metadata." + n, where n = CONTENT.version.

Here is an SQL example to get the latest version of a page's Scaffolding data:

select c.contentid, c.title, c.version, o.text_val

from CONTENT c, OS_PROPERTYENTRY o 

where c.title='My Page' and c.prevver is NULL and c.contentid=o.entity_id and o.entity_key=concat('~metadata.',c.version)