For example, search for strings like “Russia” across all posts.
Yes, possible.
On the latest post view, you can use the search filter which looks for the string inside the title and description of every post (the description is the post itself which you can see in the markdown section of the post page).
It is using this API filter
So you could do the same via the API using
curl -X 'GET' \
'https://api.obstracts.com/v1/posts/?text=russia' \
-H 'accept: application/json' \
-H 'API-KEY: HIDDEN'
Note, if you need to be more specific title and description filters do exist on the same endpoint.
p.s. A more effective way to search for locations is using the locations search 
