fix(swarm): mTLS client-cert authz on the swarm control plane

The swarm controller (port 8770) exposed 9 routes with zero app-layer
auth, and swarmctl --tls defaulted off — anyone able to reach the port
could enroll workers (minting CA-signed certs + private keys), deploy,
or tear down the fleet. Two fail-closed layers:

- require_operator_cert gates every operator route (enroll/deploy/
  teardown/hosts/check/deckies). When mTLS is on, the peer cert's CN
  must be an operator identity (decnet-master/swarmctl); worker and
  updater@* certs are rejected. Plaintext loopback (single-host master)
  is accepted as the local operator — the docker.sock boundary.
- swarmctl refuses to bind a routable interface without --tls, so a
  network-exposed plaintext control plane can never start.

/heartbeat keeps its worker fingerprint pinning. Closes the two ASVS
criticals (control-plane no-auth, unauthenticated cert minting).

decnet/web/router/swarm/api_check_hosts.py

@@ -16,6 +16,7 @@ from decnet.logging import get_logger
 from decnet.swarm.client import AgentClient
 from decnet.web.db.repository import BaseRepository
 from decnet.web.dependencies import get_repo
+from decnet.web.router.swarm._mtls import PeerCert, require_operator_cert
 from decnet.web.db.models import SwarmCheckResponse, SwarmHostHealth
 
 log = get_logger("swarm.check")
@@ -26,6 +27,7 @@ router = APIRouter()
 @router.post("/check", response_model=SwarmCheckResponse, tags=["Swarm Health"])
 async def api_check_hosts(
     repo: BaseRepository = Depends(get_repo),
+    _operator: PeerCert = Depends(require_operator_cert),
 ) -> SwarmCheckResponse:
     hosts = await repo.list_swarm_hosts()