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/public/manifest.webmanifest

@@ -0,0 +1,25 @@
+{
+  "name": "TaskStream",
+  "short_name": "TaskStream",
+  "description": "Gestión de proyectos y tickets en tiempo real",
+  "lang": "es",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "background_color": "#f7f7fa",
+  "theme_color": "#a347e1",
+  "icons": [
+    {
+      "src": "/logo-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/logo-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
+    }
+  ]
+}