feat(notify): web push as a notification channel

- VAPID keys from VAPID_* or generated once (push_keys, migration 9);
  devices in push_subscriptions, forgotten on 404/410 from the push service
- push column for every event (also due tasks) and the daily summary,
  bundles as one notification; /push/key, /devices, /subscribe,
  /unsubscribe, /test
- settings: switch push on per device, test it, see the devices; the
  service worker shows notifications and focuses MindSift on click
- tests decrypt a sent push like a browser (RFC 8291) and check VAPID

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -33,6 +33,8 @@ services:
       - NOTIFY_DIGEST_MINUTES=${NOTIFY_DIGEST_MINUTES:-5}
       - SMTP_TLS=${SMTP_TLS:-true}
       - SMTP_SKIP_VERIFY=${SMTP_SKIP_VERIFY:-false}
+      # Web-Push: ohne Angabe erzeugt MindSift die Schlüssel selbst
+      # - VAPID_SUBJECT=${VAPID_SUBJECT:-mailto:admin@example.com}
       # Impressum / Datenschutz
       - LEGAL_IMPRINT_URL=${LEGAL_IMPRINT_URL:-}
       - LEGAL_PRIVACY_URL=${LEGAL_PRIVACY_URL:-}