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>

backend/app/config.py

@@ -9,6 +9,9 @@ class Settings(BaseSettings):
     ADMIN_PASSWORD: str = "admin"
     BCCR_API_EMAIL: str = ""
     BCCR_API_TOKEN: str = ""
+    VAPID_PRIVATE_KEY: str = ""
+    VAPID_PUBLIC_KEY: str = ""
+    VAPID_CLAIM_EMAIL: str = "mailto:admin@wealth.cescalante.dev"
 
     class Config:
         env_file = ".env"