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/src/index.html

@@ -12,6 +12,8 @@
     <link rel="icon" type="image/x-icon" href="favicon.ico" />
     <link rel="icon" type="image/png" sizes="96x96" href="logo-96.png" />
     <link rel="apple-touch-icon" href="logo.png" />
+    <!-- Installable (on iPhone, push only reaches an installed web app) -->
+    <link rel="manifest" href="manifest.webmanifest" />
     <link rel="preconnect" href="https://fonts.googleapis.com" />
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
     <!-- Inter, like pireon.dev · Geist Mono only for data (ticket keys, counters, versions) -->