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/lib/ads.ts

@@ -0,0 +1,51 @@
+// The ads-network abstraction (D28). VK is the only network now; a future network for another
+// platform implements the same rewardedReady/showRewarded surface without touching the callers. On
+// the contour a build flag (VITE_ADS_STUB) substitutes a toast stub for the real ad, so the reward
+// flow is testable without waiting for a real ad — production never sets the flag, so a real ad that
+// fails to load there must not credit (no stub fallback in prod).
+import { executionContext } from './wallet';
+import { vkRewardedReady, vkShowRewarded } from './vk';
+
+/**
+ * adsStubEnabled reports the contour test stub (VITE_ADS_STUB=1). Production never sets it, so it
+ * always shows real ads. The mock e2e forces it on so the reward flow is exercisable without a VK
+ * bridge. To capture the real VK ad result on the contour (the diagnostic), deploy with the flag
+ * off first; flip it on afterwards for routine stub testing.
+ */
+export function adsStubEnabled(): boolean {
+  if (import.meta.env.MODE === 'mock') return true;
+  return (import.meta.env as Record<string, string | undefined>).VITE_ADS_STUB === '1';
+}
+
+/** RewardedResult is one rewarded-ad view: whether it was watched, the raw provider result as JSON
+ *  (forwarded for the contour diagnostic), and whether it was the test stub (so the caller can show
+ *  the "ad fired" marker toast). */
+export interface RewardedResult {
+  watched: boolean;
+  data: string;
+  stub: boolean;
+}
+
+/**
+ * rewardedReady reports whether a rewarded ad is ready to show, gating the "watch for chips" button.
+ * The stub is always ready; a real ad is ready only inside VK with preloaded material. Rewarded is
+ * VK-only (D28).
+ */
+export async function rewardedReady(): Promise<boolean> {
+  if (adsStubEnabled()) return true;
+  if (executionContext() !== 'vk') return false;
+  return vkRewardedReady();
+}
+
+/**
+ * showRewarded shows a rewarded ad and reports whether it was watched, the raw provider result, and
+ * whether it was the stub. On the stub it resolves watched immediately (the caller shows the "ad
+ * fired" toast).
+ */
+export async function showRewarded(): Promise<RewardedResult> {
+  if (adsStubEnabled()) {
+    return { watched: true, data: JSON.stringify({ stub: true }), stub: true };
+  }
+  const r = await vkShowRewarded();
+  return { watched: r.watched, data: r.data, stub: false };
+}