feat: push notifications on each device (Web Push)

- What reaches the bell also reaches the devices where the person
  turned «Avisos en este dispositivo» on, with TaskStream closed: the
  Notifier sends each new notification encrypted (aes128gcm) and signed
  with VAPID, and forgets subscriptions the push service dropped
  (404/410). A failure never blocks the event stream.
- The API keeps one subscription per device (push_subscriptions) and
  exposes the VAPID public key in the notification settings; GraphQL
  subscribePush/unsubscribePush, REST /me/push-subscriptions. Without
  VAPID_PUBLIC_KEY and VAPID_PRIVATE_KEY push is off.
- The web registers /push-sw.js, a service worker that only shows the
  notification and opens its ticket on a tap (no caching, served
  without cache by nginx), and a manifest that makes it installable,
  which iPhones need for push.
- The bell's toggles show their text next to them.

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

apps/api/src/app/config.schema.ts

@@ -52,6 +52,11 @@ export const configSchema = Joi.object({
   // E-mail copies of notifications (optional): without SMTP_URL nothing is sent.
   SMTP_URL: Joi.string().allow('').optional(),
   MAIL_FROM: Joi.string().allow('').optional(),
+  // Push notifications (optional): without both VAPID keys nothing is sent.
+  // Make them once with `npx web-push generate-vapid-keys`.
+  VAPID_PUBLIC_KEY: Joi.string().allow('').optional(),
+  VAPID_PRIVATE_KEY: Joi.string().allow('').optional(),
+  VAPID_SUBJECT: Joi.string().allow('').optional(),
   // Public address of the web app, for links in e-mails.
   WEB_URL: Joi.string().allow('').optional(),
 });