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/web/nginx.conf

@@ -28,6 +28,19 @@ server {
     proxy_read_timeout 1h;
   }
 
+  # The service worker and the manifest have no hash in their name: never
+  # cached, or browsers would keep an old one (exact matches win over the
+  # regex below).
+  location = /push-sw.js {
+    add_header Cache-Control "no-cache";
+    try_files $uri =404;
+  }
+  location = /manifest.webmanifest {
+    add_header Cache-Control "no-cache";
+    default_type application/manifest+json;
+    try_files $uri =404;
+  }
+
   # Hashed bundles can be cached forever; index.html never.
   location ~* \.(js|css|woff2?|svg|png|ico)$ {
     expires 1y;