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/infrastructure/src/lib/mail/smtp-notification.mailer.ts

@@ -0,0 +1,66 @@
+import { Injectable, Logger } from '@nestjs/common';
+import { ConfigService } from '@nestjs/config';
+import {
+  MailRecipient,
+  NotificationMailer,
+} from '@task-stream/api/tracker/application';
+import { NotificationReadModel } from '@task-stream/api/tracker/domain';
+import { createTransport, Transporter } from 'nodemailer';
+
+/**
+ * E-mail copies through SMTP_URL (`smtps://user:pass@smtp.example.org`).
+ * Without it nothing is sent and nobody is offered e-mail.
+ */
+@Injectable()
+export class SmtpNotificationMailer extends NotificationMailer {
+  private readonly logger = new Logger(SmtpNotificationMailer.name);
+  private readonly transport: Transporter | null;
+  private readonly from: string;
+  private readonly webUrl: string;
+
+  constructor(config: ConfigService) {
+    super();
+    const url = config.get<string>('SMTP_URL');
+    this.transport = url ? createTransport(url) : null;
+    this.from =
+      config.get<string>('MAIL_FROM') || 'TaskStream <no-reply@pireon.dev>';
+    this.webUrl = (config.get<string>('WEB_URL') || '').replace(/\/+$/, '');
+    if (this.transport) this.logger.log('E-mail notifications enabled');
+  }
+
+  get enabled(): boolean {
+    return this.transport !== null;
+  }
+
+  async send(
+    to: MailRecipient,
+    notification: NotificationReadModel,
+  ): Promise<void> {
+    if (!this.transport) return;
+    const link = this.linkTo(notification);
+    const subject = notification.issueKey
+      ? `[${notification.issueKey}] ${notification.summary}`
+      : `[${notification.projectKey}] ${notification.summary}`;
+    await this.transport.sendMail({
+      from: this.from,
+      to: { name: to.name, address: to.email },
+      subject: subject.length > 160 ? `${subject.slice(0, 159)}…` : subject,
+      text: [
+        `Hola ${to.name}:`,
+        '',
+        notification.summary,
+        ...(link ? ['', link] : []),
+        '',
+        '— TaskStream. Puedes dejar de recibir estos correos desde las notificaciones de la app.',
+      ].join('\n'),
+    });
+  }
+
+  private linkTo(notification: NotificationReadModel): string | null {
+    if (!this.webUrl) return null;
+    const project = encodeURIComponent(notification.projectKey);
+    return notification.issueKey
+      ? `${this.webUrl}/p/${project}/issues/${encodeURIComponent(notification.issueKey)}`
+      : `${this.webUrl}/p/${project}`;
+  }
+}