Pre-warm sessions cache for the active project

Pre-warm OpenSearch shards + Redis list cache for the active project. Fire-and-forget: returns {ok:true} immediately while the warmup runs in the background. Triggered from the frontend right before a session query is likely (sidebar hover, global search modal mount, RecipientPicker mount, periodic refresh while on the Sessions page) so the user doesn't pay the cold-shard penalty on first interaction.