feat(webhooks): subscription CRUD + HMAC-signed delivery client

Introduces the webhook egress foundation — a new WebhookSubscription
table, admin-gated CRUD under /api/v1/webhooks, and the shared
delivery client that both the test-ping route and the upcoming worker
will use. No worker yet; this commit is API + model + client only.

Simple-mode enum (AttackerDetail / DeckyStatus / SystemStatus) expands
to bus-topic patterns at the router layer; storage is always the raw
pattern list. Advanced mode lets admins supply raw NATS-style patterns
directly. Filter-at-subscribe: the worker (next commit) will subscribe
to the union of patterns across enabled subscriptions.

Delivery client handles HMAC-SHA256 signing (X-DECNET-Signature),
retry on 429/5xx/network errors with jittered backoff, no-retry on
4xx. Secrets never leave the server on GET/LIST — only the create
response carries the secret for copy-out.

CRUD routes publish WEBHOOK_SUBSCRIPTIONS_CHANGED on the bus after
every mutation so the (future) worker can hot-reload.

Opens DEBT-037 for the deferred items (circuit breaker, dead-letter,
batch delivery, payload templates, secret-at-rest).

decnet/web/router/__init__.py

@@ -33,6 +33,7 @@ from .swarm_updates import swarm_updates_router
 from .swarm_mgmt import swarm_mgmt_router
 from .system import system_router
 from .topology import topology_router
+from .webhooks import webhooks_router
 
 api_router = APIRouter(
     # Every route under /api/v1 is auth-guarded (either by an explicit
@@ -105,3 +106,6 @@ api_router.include_router(system_router)
 
 # MazeNET Topologies (nested topology CRUD + mutation queue)
 api_router.include_router(topology_router)
+
+# External webhook subscriptions (SIEM/SOAR egress)
+api_router.include_router(webhooks_router)