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/__init__.py

@@ -23,6 +23,7 @@ from .health.api_get_health import router as health_router
 from .artifacts.api_get_artifact import router as artifacts_router
 from .swarm_updates import swarm_updates_router
 from .swarm_mgmt import swarm_mgmt_router
+from .system import system_router
 
 api_router = APIRouter()
 
@@ -68,3 +69,6 @@ api_router.include_router(swarm_updates_router)
 
 # Swarm Management (dashboard: hosts, deckies, agent enrollment bundles)
 api_router.include_router(swarm_mgmt_router)
+
+# System info (deployment-mode auto-detection, etc.)
+api_router.include_router(system_router)