Add budget module and push notifications for transactions

Budget: recurring items CRUD, yearly/monthly projections with no-double-count
logic, and full UI (overview, monthly detail, recurring items manager).

Push notifications: Web Push via VAPID keys, triggered on transaction creation
from n8n. Includes service worker handlers, frontend subscription flow, and
a test button on the Dashboard (temporary).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.prod.yml

@@ -28,6 +28,8 @@ services:
       SECRET_KEY: ${SECRET_KEY}
       ADMIN_USERNAME: ${ADMIN_USERNAME}
       ADMIN_PASSWORD: ${ADMIN_PASSWORD}
+      VAPID_PRIVATE_KEY: ${VAPID_PRIVATE_KEY}
+      VAPID_PUBLIC_KEY: ${VAPID_PUBLIC_KEY}
     expose:
       - "8000"
     networks: