feat: notifications in the app and, optionally, by e-mail

Derived from events and kept per person: whoever a ticket is assigned to,
whoever reports or works on it when it moves, fails QA or gets a comment,
and whoever joins a project. Never about what you did yourself, once per
event. They arrive live through a subscription to a bell in the header,
and are copied by e-mail to whoever turns it on when SMTP_URL is set.

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

docker-compose.yml

@@ -58,6 +58,10 @@ services:
       OPENAPI_FILE: ''
       ATTACHMENTS_DIR: /data/attachments
       ATTACHMENTS_MAX_MB: ${ATTACHMENTS_MAX_MB:-25}
+      # E-mail copies of notifications (optional).
+      SMTP_URL: ${SMTP_URL:-}
+      MAIL_FROM: ${MAIL_FROM:-}
+      WEB_URL: ${WEB_URL:-}
     volumes:
       # Files attached to tickets: lose this volume and the tickets point at nothing.
       - attachments-data:/data/attachments