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>

package.json

@@ -64,6 +64,7 @@
     "rxjs": "^7.8.0",
     "tslib": "^2.8.1",
     "typeorm": "^0.3.31",
+    "web-push": "3.6.7",
     "ws": "^8.22.0",
     "zod": "^4.6.5"
   },
@@ -101,6 +102,7 @@
     "@types/jest": "~30.0.0",
     "@types/node": "^22.0.0",
     "@types/nodemailer": "^8.0.2",
+    "@types/web-push": "3.6.4",
     "@types/ws": "^8.18.2",
     "@typescript-eslint/utils": "^8.58.0",
     "@vitest/coverage-v8": "~4.1.0",