feat(payments): payment-event dispatcher + the provider-return UX

Deliver payment_events to connected clients as an in-app wallet-refresh push: a
background dispatcher drains undispatched events and publishes a KindNotification
"payment" signal, marking each delivered; the client bumps a wallet-refresh
counter the open Wallet screen watches, re-fetching in place. A return-focus
refetch is the fallback. The Robokassa Success/Fail return now serves a
self-closing page (the payment opens in a separate window) so the customer drops
back into the live app instead of a cold start. Integration test for the event
drain/mark queue.

backend/internal/notify/notify.go

@@ -73,6 +73,10 @@ const (
 	// (e.g. an email was confirmed via the one-tap deeplink opened in another browser),
 	// so it re-fetches profile.get. It carries no payload. In-app only.
 	NotifyProfile = "profile"
+	// NotifyPayment tells the client that the viewer's wallet changed from a payment-intake event
+	// (a credit or a refund), so it re-fetches the wallet in place. It carries no payload. In-app
+	// only; the payment_events dispatcher emits it after the crediting transaction commits.
+	NotifyPayment = "payment"
 	// NotifyUserBlocked confirms to the blocker that a per-user block took effect,
 	// carrying the blocked account, so every one of the blocker's sessions updates the
 	// in-game block/add-friend controls and the struck name in place. It is delivered