feat(webhook): worker + CLI + systemd unit

Introduces the `decnet webhook` long-running worker that consumes the
internal bus and POSTs matching events to configured subscriptions.

Design: one task per (subscription, pattern) pair. Each task opens
its own bus subscription, iterates events, and dispatches via the
shared deliver() client. No intermediate queue, no in-memory filter
matching — the bus's own pattern matcher is the filter. Reloads on
`system.webhook.subscriptions_changed` signals from the CRUD router,
with a 60s fallback timer in case a signal is lost.

Shutdown propagates via CancelledError on the outer task; all inner
subscription tasks are cancelled and awaited in a finally block.
Bus unavailable → worker stays up in idle mode per the DEBT-031
pattern, logging one warning.

Registered as a master-only CLI command (agents don't configure
webhooks — the subscription store lives on master). systemd unit
mirrors the profiler template; added to decnet.target Wants= list so
`systemctl start decnet.target` brings it up alongside everything
else. `decnet init` auto-picks up the new .service.j2 via its
existing `glob("decnet-*.service.j2")` sweep.

decnet/cli/gating.py

@@ -29,7 +29,7 @@ MASTER_ONLY_COMMANDS: frozenset[str] = frozenset({
     "api", "swarmctl", "deploy", "redeploy", "teardown",
     "mutate", "listener", "profiler",
     "services", "distros", "correlate", "archetypes", "web",
-    "db-reset", "init",
+    "db-reset", "init", "webhook",
 })
 MASTER_ONLY_GROUPS: frozenset[str] = frozenset({"swarm", "topology", "geoip"})