feat: outgoing webhooks

Owners and admins can point a project at other systems: a signed JSON
POST (HMAC-SHA256 of the body) when tickets, comments, releases or sprints
change, filtered by event or family. Deliveries are retried a couple of
times, the last one is shown, and a ping can be sent from the settings.
Private addresses are refused unless WEBHOOKS_ALLOW_PRIVATE is set.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -45,6 +45,9 @@ SMTP_URL=
 MAIL_FROM=
 WEB_URL=http://localhost:4200
 
+# Outgoing webhooks refuse hosts on this machine or its private network unless true.
+WEBHOOKS_ALLOW_PRIVATE=false
+
 # Docker builds only: token of Pireon's Nexus for @pi/ui (build secret, never stored in an image).
 # Locally: export NEXUS_TOKEN=$(grep '^//nexus.pireon.dev/repository/npm-releases/:_authToken=' ~/.npmrc | cut -d= -f2-)
 NEXUS_TOKEN=