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/inttest/payments_intake_test.go

@@ -240,6 +240,100 @@ func TestPaymentsTelegramPreCheckoutDeclines(t *testing.T) {
 	}
 }
 
+// setRewardConfig sets the rewarded payout and caps on the shared config row, restoring the defaults
+// after the test (the row is a singleton shared across the sequential suite).
+func setRewardConfig(t *testing.T, payout, dailyCap, hourlyCap int) {
+	t.Helper()
+	if _, err := testDB.ExecContext(context.Background(),
+		`UPDATE payments.config SET rewarded_payout_chips=$1, reward_daily_cap=$2, reward_hourly_cap=$3`,
+		payout, dailyCap, hourlyCap); err != nil {
+		t.Fatalf("set reward config: %v", err)
+	}
+	t.Cleanup(func() {
+		_, _ = testDB.ExecContext(context.Background(),
+			`UPDATE payments.config SET rewarded_payout_chips=0, reward_daily_cap=50, reward_hourly_cap=10`)
+	})
+}
+
+// TestPaymentsRewardCredit exercises the rewarded-video credit: a watched view credits the VK segment
+// the configured payout, a retried view (same nonce) credits once, and the hourly cap blocks the
+// third distinct view.
+func TestPaymentsRewardCredit(t *testing.T) {
+	ctx := context.Background()
+	svc := newPaymentsService()
+	acc := uuid.New()
+	setRewardConfig(t, 5, 5, 2) // 5 chips/view, daily 5, hourly 2
+	vk := payments.NewContext("vk", "web")
+	present := []payments.Source{payments.SourceVK}
+
+	out, err := svc.CreditReward(ctx, acc, vk, present, "n1")
+	if err != nil {
+		t.Fatalf("credit: %v", err)
+	}
+	if out.Chips != 5 || out.Capped {
+		t.Fatalf("reward outcome = %+v, want 5 chips, not capped", out)
+	}
+	if got := readBalance(t, acc, "vk"); got != 5 {
+		t.Errorf("vk balance = %d, want 5", got)
+	}
+
+	// A retried view (same nonce) credits nothing more.
+	out2, err := svc.CreditReward(ctx, acc, vk, present, "n1")
+	if err != nil {
+		t.Fatalf("retry: %v", err)
+	}
+	if !out2.AlreadyCredited {
+		t.Error("retried view not flagged AlreadyCredited")
+	}
+	if got := readBalance(t, acc, "vk"); got != 5 {
+		t.Errorf("vk balance after retry = %d, want 5 (credited once)", got)
+	}
+
+	// A second distinct view credits again (2 total).
+	if _, err := svc.CreditReward(ctx, acc, vk, present, "n2"); err != nil {
+		t.Fatalf("credit 2: %v", err)
+	}
+	if got := readBalance(t, acc, "vk"); got != 10 {
+		t.Errorf("vk balance = %d, want 10", got)
+	}
+
+	// The third distinct view hits the hourly cap (2) — capped, no credit.
+	out3, err := svc.CreditReward(ctx, acc, vk, present, "n3")
+	if err != nil {
+		t.Fatalf("credit 3: %v", err)
+	}
+	if !out3.Capped {
+		t.Error("third view not capped (hourly cap 2)")
+	}
+	if got := readBalance(t, acc, "vk"); got != 10 {
+		t.Errorf("vk balance after cap = %d, want 10 (capped view credited nothing)", got)
+	}
+}
+
+// TestPaymentsRewardDisabledAndContext verifies rewarded is inert when unconfigured (0 payout) and
+// refused outside a VK context (rewarded is VK-only, D28).
+func TestPaymentsRewardDisabledAndContext(t *testing.T) {
+	ctx := context.Background()
+	svc := newPaymentsService()
+	acc := uuid.New()
+	setRewardConfig(t, 0, 50, 10) // payout 0 = disabled
+
+	vk := payments.NewContext("vk", "web")
+	out, err := svc.CreditReward(ctx, acc, vk, []payments.Source{payments.SourceVK}, "d1")
+	if err != nil {
+		t.Fatalf("credit (disabled): %v", err)
+	}
+	if out.Chips != 0 || out.Capped || out.AlreadyCredited {
+		t.Fatalf("disabled reward outcome = %+v, want 0 chips, not capped", out)
+	}
+
+	// A direct (non-VK) context is refused — rewarded is VK-only.
+	direct := payments.NewContext("direct", "web")
+	if _, err := svc.CreditReward(ctx, acc, direct, []payments.Source{payments.SourceDirect}, "d2"); !errors.Is(err, payments.ErrUntrusted) {
+		t.Fatalf("direct-context reward = %v, want ErrUntrusted", err)
+	}
+}
+
 // TestPaymentsFundAmountMismatch verifies a callback whose paid amount does not match the order is
 // refused and credits nothing (§9: verify the amount after matching by order id).
 func TestPaymentsFundAmountMismatch(t *testing.T) {