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/bus/topics.py

@@ -93,6 +93,12 @@ SYSTEM_CONTROL = "control"
 WORKER_CONTROL_STOP = "stop"
 WORKER_CONTROL_START = "start"
 
+# Webhook subscription-set changed — published by the CRUD router after any
+# create / update / delete on WebhookSubscription so the webhook worker can
+# reload its in-memory subscription list and re-subscribe to the new union
+# of patterns. Payload is currently empty; consumers only need the signal.
+WEBHOOK_SUBSCRIPTIONS_CHANGED = "system.webhook.subscriptions_changed"
+
 
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