Worker stderr is now inherited by the parent, so its log lines land in the
server log — but they arrived with the default logging format and a noisy
"preview-worker config=..." line at every spawn.
- Extract the emoji level-prefix formatting from cista.sanic_logging into
cista.util.logformat, which has no Sanic dependency (the worker must not
import Sanic: import-time prints could corrupt the stdout protocol).
- Worker configures its stderr handler with the same emoji prefixes plus a
worker[pid] tag, and the config-loaded info message is removed.
