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/__init__.py

@@ -37,6 +37,7 @@ from . import (
     topology,
     updater,
     web,
+    webhook,
     workers,
 )
 from .gating import _gate_commands_by_mode
@@ -54,7 +55,7 @@ for _mod in (
     swarm,
     deploy, lifecycle, workers, inventory,
     web, profiler, sniffer, db,
-    topology, bus, geoip, init,
+    topology, bus, geoip, init, webhook,
 ):
     _mod.register(app)