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>

libs/web/tracker/domain/src/lib/models/enums.ts

@@ -38,3 +38,15 @@ export enum ReleaseStatus {
   RELEASED = 'RELEASED',
   ARCHIVED = 'ARCHIVED',
 }
+
+/** Why someone is told. */
+export enum NotificationKind {
+  ASSIGNED = 'ASSIGNED',
+  STATUS_CHANGED = 'STATUS_CHANGED',
+  QA_FAILED = 'QA_FAILED',
+  COMMENTED = 'COMMENTED',
+  MENTIONED = 'MENTIONED',
+  WATCHED = 'WATCHED',
+  ADDED_TO_PROJECT = 'ADDED_TO_PROJECT',
+  REMINDER = 'REMINDER',
+}