feat(fleet): auto-swarm deploy — shard across enrolled workers when master

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.

decnet/web/router/system/__init__.py

@@ -0,0 +1,6 @@
+from fastapi import APIRouter
+
+from .api_deployment_mode import router as deployment_mode_router
+
+system_router = APIRouter(prefix="/system", tags=["System"])
+system_router.include_router(deployment_mode_router)