feat: Add push notification support with VAPID keys

- Backend: web-push integration with VAPID keys
- Push subscription endpoints (GET /api/push/public-key, POST /api/push/subscribe)
- In-memory subscription store
- Frontend: Auto-subscribe to push after notification permission granted
- Service Worker: Push event handler
- Send push notifications when torrents complete
- Works even when browser is closed

backend/Cargo.toml

@@ -26,3 +26,5 @@ thiserror = "2.0.18"
 dotenvy = "0.15.7"
 utoipa = { version = "5.4.0", features = ["axum_extras"] }
 utoipa-swagger-ui = { version = "9.0.2", features = ["axum"] }
+web-push = "0.10"
+base64 = "0.22"