I think you need to update manual script in this way:
DROP SCHEMA public CASCADE;
CREATE SCHEMA public;
ALTER SCHEMA public OWNER TO discourse;
CREATE EXTENSION IF NOT EXISTS hstore;
CREATE EXTENSION IF NOT EXISTS pg_trgm;
END;
There is no need to CREATE SCHEMA public (and set owner), cause newer Pg creates schema from SQL backup dump.
However table permissions must be fixed to discourse user, by default they are postgres.
Now it works! Thank you all for ideas and support.