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>

docker-compose.yml

@@ -63,6 +63,10 @@ services:
       SMTP_URL: ${SMTP_URL:-}
       MAIL_FROM: ${MAIL_FROM:-}
       WEB_URL: ${WEB_URL:-}
+      # Push notifications (optional): `npx web-push generate-vapid-keys`.
+      VAPID_PUBLIC_KEY: ${VAPID_PUBLIC_KEY:-}
+      VAPID_PRIVATE_KEY: ${VAPID_PRIVATE_KEY:-}
+      VAPID_SUBJECT: ${VAPID_SUBJECT:-}
       # Outgoing webhooks to private addresses (off: they could reach internal services).
       WEBHOOKS_ALLOW_PRIVATE: ${WEBHOOKS_ALLOW_PRIVATE:-false}
       # Due-date reminders, checked every hour.