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/scrabble.fbs

@@ -866,6 +866,9 @@ table Wallet {
   ads_forever:bool;
   ads_paid_until_ms:long;
   hints:int;
+  // reward_chips is the chips a rewarded-video view earns in the current context (0 = unavailable
+  // here — outside VK, or unconfigured); the client shows the "watch for chips" button only when > 0.
+  reward_chips:int;
 }
 
 // WalletBuyRequest buys a chip-priced value with chips: the product to buy.
@@ -873,6 +876,13 @@ table WalletBuyRequest {
   product_id:string;
 }
 
+// WalletRewardRequest credits a watched rewarded video: nonce is the per-view idempotency key (a
+// retry credits once); diag carries the raw VK ad result for a temporary contour diagnostic log.
+table WalletRewardRequest {
+  nonce:string;
+  diag:string;
+}
+
 // WalletOrderRequest opens a money order to fund a chip pack: the pack to buy.
 table WalletOrderRequest {
   product_id:string;