Expose active DB Alembic schema version through the REST API

[rtamalin]

Similarly to how we expose the package version, provide a REST API query (/schem-version) which returns the active Alembic schema version, which can be used to confirm/validate that schema migrations have been applied.

Potentially implement a means, via the internal DB API, by which the expected schema version for the DB can be determined programmatically to allow automated checking that the schema is up-to-date.