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/api/tracker/application/src/testing/in-memory.ts

@@ -170,6 +170,16 @@ export class FakeUserDirectory extends UserDirectory {
     return this.users.get(userId) ?? null;
   }
 
+  async contacts(userIds: string[]) {
+    return userIds
+      .filter((id) => this.users.has(id))
+      .map((id) => ({
+        id,
+        name: this.users.get(id)?.name ?? id,
+        email: `${id}@example.org`,
+      }));
+  }
+
   async refs(): Promise<UserRefs> {
     const of = (id: string) => ({ id, name: this.users.get(id)?.name ?? id });
     return { of, ofNullable: (id) => (id ? of(id) : null) };