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>

.env.example

@@ -40,6 +40,11 @@ OPENAPI_FILE=apps/api/openapi.json
 ATTACHMENTS_DIR=data/attachments
 ATTACHMENTS_MAX_MB=25
 
+# Optional e-mail copies of notifications. Without SMTP_URL nothing is sent.
+SMTP_URL=
+MAIL_FROM=
+WEB_URL=http://localhost:4200
+
 # 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=