Improve saving state

**Motivation**
We could improve both readability and performance when saving states by getting rid of the preliminary select.

**Acceptance criteria**
By applying the following approach (we gain one roundtrip to the database):
* if tag is Latest and rev == 1: INSERT command
* if tag is Latest and rev >1: UPDATE command
* If tag is a custom tag: INSERT command with a ON CONFLICT clause

Locate the state-saving implementation and identify the preliminary database select and existing revision/tag handling. Verify the INSERT, UPDATE, and ON CONFLICT paths against the acceptance criteria, then confirm that saving a state uses one fewer database roundtrip.

Written by the indexing model from the issue text.