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>

.env.example

@@ -48,6 +48,13 @@ SMTP_URL=
 MAIL_FROM=
 WEB_URL=http://localhost:4200
 
+# Optional push notifications (Web Push). Without both keys nothing is sent.
+# Make them once with `npx web-push generate-vapid-keys` (changing them drops
+# every device's subscription). VAPID_SUBJECT: a mailto: or https: contact.
+VAPID_PUBLIC_KEY=
+VAPID_PRIVATE_KEY=
+VAPID_SUBJECT=mailto:no-reply@pireon.dev
+
 # Outgoing webhooks refuse hosts on this machine or its private network unless true.
 WEBHOOKS_ALLOW_PRIVATE=false