fix(payments): VK-iOS freeze is purchase-only; remove the rewarded diagnostic

Testing the rewarded slice surfaced a contradiction: rewarded ads let a VK-iOS
user earn vk chips, but the blanket VK-iOS "spend freeze" then blocked spending
them (the wallet showed "5 (view-only)"). Apple's ToS forbids only BUYING in-app
values on VK-iOS (money -> chips), not spending or earning them — so the freeze is
corrected to purchase-only: vkFrozen() now gates only CreateOrder (the money-in
step), not spendableSources (spending). VK-wallet chips — earned via rewarded ads
or bought on the same account elsewhere (VK Android) — now spend on VK-iOS too.
(Owner's ToS finding.)

Also: the temporary contour diagnostic confirmed VK returns only {result:true} for
a rewarded view (no token/signature) — client-attested is final, no hardening
possible — so the diagnostic (the log and the diag wire field) is removed.

Docs: PAYMENTS(+ru) VK-iOS freeze + D17 amend + PLAN E6. Tests updated (gate /
wallet VK-iOS now spendable).

gateway/internal/backendclient/api.go

@@ -384,18 +384,16 @@ type walletBuyBody struct {
 	ProductID string `json:"product_id"`
 }
 
-// walletRewardBody is the rewarded-video credit request: the per-view idempotency nonce and the raw
-// VK ad result forwarded for the temporary contour diagnostic.
+// walletRewardBody is the rewarded-video credit request: the per-view idempotency nonce.
 type walletRewardBody struct {
 	Nonce string `json:"nonce"`
-	Diag  string `json:"diag"`
 }
 
 // WalletReward credits a watched rewarded video and returns the updated wallet (client-attested + a
-// backend daily cap). A reached cap or unconfigured payout surfaces as an APIError domain code.
-func (c *Client) WalletReward(ctx context.Context, userID, nonce, diag string) (WalletResp, error) {
+// backend daily/hourly cap). A reached cap or unconfigured payout surfaces as an APIError domain code.
+func (c *Client) WalletReward(ctx context.Context, userID, nonce string) (WalletResp, error) {
 	var out WalletResp
-	err := c.do(ctx, http.MethodPost, "/api/v1/user/wallet/reward", userID, "", walletRewardBody{Nonce: nonce, Diag: diag}, &out)
+	err := c.do(ctx, http.MethodPost, "/api/v1/user/wallet/reward", userID, "", walletRewardBody{Nonce: nonce}, &out)
 	return out, err
 }