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.yml

@@ -23,6 +23,8 @@ services:
     container_name: wealthysmart-backend-dev
     environment:
       DATABASE_URL: postgresql://wealthy_user:wealthy_pass@db:5432/wealthysmart
+      VAPID_PRIVATE_KEY: ${VAPID_PRIVATE_KEY:-}
+      VAPID_PUBLIC_KEY: ${VAPID_PUBLIC_KEY:-}
     ports:
       - "8001:8000"
     volumes: