address follow-ups from https://github.com/authzed/spicedb/pull/2782

• We should enable this:

https://github.com/authzed/spicedb/blob/cbdf06cbf3155495a05c7038f08e6779b6e089ad/magefiles/test.go#L257-L266

• We should update https://github.com/authzed/spicedb/blob/ef1a01bda3985d312d3b63fd83c20d01a1cb40d5/internal/datastore/postgres/README.md to mention pgbouncer and the pg_hint_plan extension
• We should turn on pg_hint_plan extension for the tests
• TestCheckPermissionOnTesterNoFlakes should be a unit test that doesn't need docker
• We should have autovacuum=off in all the postgres container tests