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/postgres/migrations/00013_reward_daily_cap.sql

@@ -0,0 +1,22 @@
+-- Rewarded-video caps: the anti-abuse ceilings on free rewarded credits per user, per
+-- day and per hour. VK Mini App ads expose only a client-side watch result (no
+-- server-to-server verify), so a rewarded credit is client-attested; the caps bound a
+-- forger who skips the ad and calls the credit endpoint directly (the daily cap bounds the
+-- total; the hourly cap smooths a burst). Chips-per-view already exists
+-- (rewarded_payout_chips, default 0 = rewarded inert until the owner sets it). All are
+-- config, tuned in the admin without a release. Additive columns only — applies forward via
+-- goose with no data rewrite (no contour wipe), and an image rollback ignores them.
+-- +goose Up
+
+ALTER TABLE payments.config
+    ADD COLUMN reward_daily_cap integer DEFAULT 50 NOT NULL,
+    ADD COLUMN reward_hourly_cap integer DEFAULT 10 NOT NULL;
+ALTER TABLE payments.config
+    ADD CONSTRAINT config_reward_daily_cap_chk CHECK (reward_daily_cap >= 0),
+    ADD CONSTRAINT config_reward_hourly_cap_chk CHECK (reward_hourly_cap >= 0);
+
+-- +goose Down
+
+ALTER TABLE payments.config
+    DROP COLUMN reward_daily_cap,
+    DROP COLUMN reward_hourly_cap;