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).

ui/src/gen/fbs/scrabblefb/wallet.ts

@@ -48,8 +48,13 @@ hints():number {
   return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
 }
 
+rewardChips():number {
+  const offset = this.bb!.__offset(this.bb_pos, 12);
+  return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
+}
+
 static startWallet(builder:flatbuffers.Builder) {
-  builder.startObject(4);
+  builder.startObject(5);
 }
 
 static addSegments(builder:flatbuffers.Builder, segmentsOffset:flatbuffers.Offset) {
@@ -80,17 +85,22 @@ static addHints(builder:flatbuffers.Builder, hints:number) {
   builder.addFieldInt32(3, hints, 0);
 }
 
+static addRewardChips(builder:flatbuffers.Builder, rewardChips:number) {
+  builder.addFieldInt32(4, rewardChips, 0);
+}
+
 static endWallet(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createWallet(builder:flatbuffers.Builder, segmentsOffset:flatbuffers.Offset, adsForever:boolean, adsPaidUntilMs:bigint, hints:number):flatbuffers.Offset {
+static createWallet(builder:flatbuffers.Builder, segmentsOffset:flatbuffers.Offset, adsForever:boolean, adsPaidUntilMs:bigint, hints:number, rewardChips:number):flatbuffers.Offset {
   Wallet.startWallet(builder);
   Wallet.addSegments(builder, segmentsOffset);
   Wallet.addAdsForever(builder, adsForever);
   Wallet.addAdsPaidUntilMs(builder, adsPaidUntilMs);
   Wallet.addHints(builder, hints);
+  Wallet.addRewardChips(builder, rewardChips);
   return Wallet.endWallet(builder);
 }
 }