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).

pkg/fbs/scrabblefb/Wallet.go

@@ -97,8 +97,20 @@ func (rcv *Wallet) MutateHints(n int32) bool {
 	return rcv._tab.MutateInt32Slot(10, n)
 }
 
+func (rcv *Wallet) RewardChips() int32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(12))
+	if o != 0 {
+		return rcv._tab.GetInt32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *Wallet) MutateRewardChips(n int32) bool {
+	return rcv._tab.MutateInt32Slot(12, n)
+}
+
 func WalletStart(builder *flatbuffers.Builder) {
-	builder.StartObject(4)
+	builder.StartObject(5)
 }
 func WalletAddSegments(builder *flatbuffers.Builder, segments flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(segments), 0)
@@ -115,6 +127,9 @@ func WalletAddAdsPaidUntilMs(builder *flatbuffers.Builder, adsPaidUntilMs int64)
 func WalletAddHints(builder *flatbuffers.Builder, hints int32) {
 	builder.PrependInt32Slot(3, hints, 0)
 }
+func WalletAddRewardChips(builder *flatbuffers.Builder, rewardChips int32) {
+	builder.PrependInt32Slot(4, rewardChips, 0)
+}
 func WalletEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }