Deckies merged in from a prior deployment's saved state kept their
original host_uuid — which dispatch_decnet_config then 404'd on if that
host had since been decommissioned or re-enrolled at a different uuid.
Before round-robin assignment, drop any host_uuid that isn't in the live
swarm_hosts set so orphaned entries get reassigned instead of exploding
with 'unknown host_uuid'.
POST /deckies/deploy now branches on DECNET_MODE + enrolled host presence:
when the caller is a master with at least one reachable swarm host, round-
robin host_uuids are assigned over new deckies and the config is dispatched
via AgentClient. Falls back to local docker-compose otherwise.
Extracts the dispatch loop from api_deploy_swarm into dispatch_decnet_config
so both endpoints share the same shard/dispatch/persist path. Adds
GET /system/deployment-mode for the UI to show 'will shard across N hosts'
vs 'will deploy locally' before the operator clicks deploy.
