## Why
The app server is long-lived, but its shared model cache otherwise
refreshes only when a caller needs it. Once the five-minute cache
expires, starting a thread or calling `model/list` can wait for
`/models` on the request path.
Refresh the cache in the background before it expires so foreground
callers normally use fresh local state.
## What changed
- Start an app-server worker that refreshes models immediately and then
every three minutes using the existing models-manager API.
- Hold only a weak reference to the models manager between refreshes, so
the worker does not extend its lifetime.
- Stop scheduling refreshes when the app-server lifecycle handle is shut
down or dropped. A refresh already in progress is allowed to finish.
- Adjust affected app-server test fixtures to distinguish the background
`/models` probe from the connection they are testing.
The existing models-manager cache, refresh strategies, auth handling,
ETag behavior, and concurrency semantics are unchanged.
## Testing
-
`models_refresh_worker::tests::refreshes_immediately_periodically_and_stops_when_dropped`
-
`suite::v2::remote_control::listen_off_honors_persisted_remote_control_enable`
-
`suite::v2::attestation::attestation_generate_round_trip_adds_header_to_responses_websocket_handshake`
jif