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).

backend/internal/payments/service_intake.go

@@ -135,6 +135,36 @@ func (s *Service) ValidatePreCheckout(ctx context.Context, orderID uuid.UUID, am
 	return PreCheckoutOutcome{OK: true, AccountID: ord.accountID}, nil
 }
 
+// providerVKAds tags a rewarded-video credit from the VK ads network in the ledger (distinct from
+// the "vk" Votes-purchase provider), so the daily cap counts only ad credits and the report separates
+// them.
+const providerVKAds = "vk_ads"
+
+// RewardPayout reports the chips a rewarded-video view earns in the caller's context — the config
+// payout in a trusted VK context with the VK segment attached, and 0 everywhere else (rewarded is
+// VK-only, D28). The client uses it to gate the "watch for chips" button.
+func (s *Service) RewardPayout(ctx context.Context, cxt Context, present []Source) (int, error) {
+	if cxt.Kind != SourceVK || !cxt.Trusted() || !has(present, SourceVK) {
+		return 0, nil
+	}
+	payout, _, _, err := s.store.rewardConfig(ctx)
+	return payout, err
+}
+
+// CreditReward credits a rewarded-video view's chips to the VK segment, client-attested (VK Mini App
+// ads expose no server verify — the client's watch result is trusted for an honest user; a forger who
+// skips the ad and calls the endpoint is bounded by the config daily cap). It is idempotent on the
+// client nonce and order-less. It credits nothing when rewarded is unconfigured (0 payout) or the
+// daily cap is reached. Rewarded video is VK-only (D28) and is an ad view — not a purchase — so the
+// VK-iOS purchase freeze does not apply; it requires a trusted VK context with the VK segment
+// attached.
+func (s *Service) CreditReward(ctx context.Context, accountID uuid.UUID, cxt Context, present []Source, nonce string) (RewardOutcome, error) {
+	if cxt.Kind != SourceVK || !cxt.Trusted() || !has(present, SourceVK) {
+		return RewardOutcome{}, ErrUntrusted
+	}
+	return s.store.creditReward(ctx, accountID, SourceVK, providerVKAds, nonce, s.clock())
+}
+
 // ExpireOrders marks pending orders older than the configured lifetime as expired, returning how
 // many were swept. It backs the periodic pending reaper; expiry is cosmetic (a late valid callback
 // still credits — see Fund).