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/screens/Wallet.svelte

@@ -10,6 +10,8 @@
   import { onExternalLinkClick, openExternalUrl } from '../lib/links';
   import { vkPlatform, vkShowOrderBox } from '../lib/vk';
   import { telegramOpenInvoice } from '../lib/telegram';
+  import { rewardedReady, showRewarded } from '../lib/ads';
+  import { GatewayError } from '../lib/client';
   import type { Wallet, Catalog, CatalogProduct } from '../lib/model';
 
   // The Wallet section: the context-visible chip balances, the active benefits, and the storefront.
@@ -41,6 +43,41 @@
   const values = $derived(catalog?.products.filter((p) => p.kind === 'value') ?? []);
   const packs = $derived(catalog?.products.filter((p) => p.kind === 'pack') ?? []);
 
+  // Rewarded video (VK ads): the "watch for chips" CTA shows only when the server offers a payout in
+  // this context (wallet.rewardChips > 0 — VK, configured) and an ad is ready to show.
+  let rewardReady = $state(false);
+  const canReward = $derived(!!wallet && wallet.rewardChips > 0 && rewardReady);
+
+  // onWatchAd shows a rewarded ad and, if watched, credits the payout server-side (client-attested +
+  // a server daily/hourly cap). A stub view (contour test) shows an "ad fired" marker; a reached cap
+  // is surfaced as its own toast, distinct from a generic error.
+  async function onWatchAd() {
+    if (busy) return;
+    busyId = 'reward';
+    try {
+      const res = await showRewarded();
+      if (res.stub) showToast('ad fired');
+      if (!res.watched) {
+        showToast(t('wallet.rewardFailed'));
+        return;
+      }
+      const nonce = crypto.randomUUID();
+      const before = wallet?.segments.find((s) => s.source === 'vk')?.chips ?? 0;
+      const w = await gateway.walletReward(nonce, res.data);
+      wallet = w;
+      const gained = (w.segments.find((s) => s.source === 'vk')?.chips ?? 0) - before;
+      showToast(gained > 0 ? t('wallet.rewardCredited', { n: gained }) : t('wallet.rewardCredited', { n: w.rewardChips }));
+    } catch (e) {
+      if (e instanceof GatewayError && e.code === 'reward_capped') {
+        showToast(t('wallet.rewardCapped'));
+      } else {
+        handleError(e);
+      }
+    } finally {
+      busyId = null;
+    }
+  }
+
   async function load() {
     try {
       const [w, c] = await Promise.all([gateway.wallet(), gateway.catalog()]);
@@ -54,6 +91,9 @@
   }
   onMount(() => {
     void load();
+    // Probe rewarded-ad readiness once so the "watch for chips" button only shows when a view would
+    // actually play.
+    void rewardedReady().then((r) => (rewardReady = r));
     // Fallback to the payment push: re-fetch when the tab regains focus, i.e. the user returned
     // from the provider's payment window.
     const onVisible = () => {
@@ -171,6 +211,14 @@
       <!-- prettier-ignore -->
       <p class="ios-note" data-testid="ios-note">{t('wallet.iosBlockedPre')}<a href={siteRoot} target="_blank" rel="noopener noreferrer" onclick={onExternalLinkClick}>{t('wallet.iosBlockedLink')}</a>{t('wallet.iosBlockedPost')}</p>
     {/if}
+    {#if canReward}
+      <div class="row reward" data-testid="reward-row">
+        <span class="name">{t('wallet.rewardWatch', { n: wallet?.rewardChips ?? 0 })}</span>
+        <button class="buy" class:working={busyId === 'reward'} data-testid="watch-ad" disabled={busy} onclick={onWatchAd}
+          >{t('wallet.rewardCta')}</button
+        >
+      </div>
+    {/if}
     {#each values as p (p.productId)}
       <div class="row product" data-testid="product" data-kind="value" data-pid={p.productId}>
         <span class="name">{p.title}</span>