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>

docker-compose.yml

@@ -62,6 +62,8 @@ services:
       SMTP_URL: ${SMTP_URL:-}
       MAIL_FROM: ${MAIL_FROM:-}
       WEB_URL: ${WEB_URL:-}
+      # Outgoing webhooks to private addresses (off: they could reach internal services).
+      WEBHOOKS_ALLOW_PRIVATE: ${WEBHOOKS_ALLOW_PRIVATE:-false}
     volumes:
       # Files attached to tickets: lose this volume and the tickets point at nothing.
       - attachments-data:/data/attachments