feat(ads): rewarded video (VK) — client-attested credit + daily/hourly caps

The first ads slice: a voluntary rewarded video credits chips. VK Mini App ads
(VKWebAppShowNativeAds) expose only a client-side watch result — no
server-to-server verify — so the credit is client-attested, guarded by a server
daily + hourly cap (config reward_daily_cap / reward_hourly_cap, default 50 / 10).
The caps are both anti-abuse (bounding a forger who skips the ad and calls the
endpoint directly) and an economic conversion lever (limiting free chips so a
player who wants more buys). D29 amended to VK's reality.

Backend: CreditReward (VK-only, order-less, idempotent on a client nonce, floored
by the caps; payout from config rewarded_payout_chips, default 0 = off) + the
wallet.reward edge op returning the updated wallet (reward_chips gates the
"watch for chips" CTA). Additive migration (two config columns).

Client: the ads-network abstraction (lib/ads.ts, VK impl) + the VK bridge
(vkRewardedReady / vkShowRewarded) + the Wallet CTA + i18n. A contour test stub
(VITE_ADS_STUB -> a toast instead of a real ad; prod always real) and a temporary
diagnostic that logs the raw VK data, so we confirm on the contour exactly what
VK returns (harden to signature-verify if it carries one).

Tests: backend integration (credit, nonce idempotency, hourly cap, disabled,
non-VK refusal) + codec unit (reward wire). Docs: PAYMENTS(+ru) §10, D29 amend,
PLAN E6. Bundle: shared budget 30->31 (reward i18n strings).

gateway/internal/transcode/transcode.go

@@ -56,6 +56,7 @@ const (
 	MsgWalletCatalog        = "wallet.catalog"
 	MsgWalletBuy            = "wallet.buy"
 	MsgWalletOrder          = "wallet.order"
+	MsgWalletReward         = "wallet.reward"
 )
 
 // Request is one decoded Execute call.
@@ -113,6 +114,7 @@ func NewRegistry(backend *backendclient.Client, tg TelegramValidator, opts ...Op
 	r.ops[MsgWalletCatalog] = Op{Handler: walletCatalogHandler(backend), Auth: true}
 	r.ops[MsgWalletBuy] = Op{Handler: walletBuyHandler(backend), Auth: true}
 	r.ops[MsgWalletOrder] = Op{Handler: walletOrderHandler(backend, nil), Auth: true}
+	r.ops[MsgWalletReward] = Op{Handler: walletRewardHandler(backend), Auth: true}
 	r.ops[MsgBlockStatus] = Op{Handler: blockStatusHandler(backend), Auth: true}
 	r.ops[MsgGameSubmitPlay] = Op{Handler: submitPlayHandler(backend), Auth: true}
 	r.ops[MsgGameState] = Op{Handler: gameStateHandler(backend), Auth: true}
@@ -379,6 +381,19 @@ func walletOrderHandler(backend *backendclient.Client, minter InvoiceMinter) Han
 	}
 }
 
+// walletRewardHandler credits a watched rewarded video and returns the updated wallet. The nonce is
+// the client's per-view idempotency key; diag carries the raw VK ad result for the contour diagnostic.
+func walletRewardHandler(backend *backendclient.Client) Handler {
+	return func(ctx context.Context, req Request) ([]byte, error) {
+		in := fb.GetRootAsWalletRewardRequest(req.Payload, 0)
+		w, err := backend.WalletReward(ctx, req.UserID, string(in.Nonce()), string(in.Diag()))
+		if err != nil {
+			return nil, err
+		}
+		return encodeWallet(w), nil
+	}
+}
+
 func blockStatusHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		bs, err := backend.BlockStatus(ctx, req.UserID)