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).

backend/internal/payments/wallet_test.go

@@ -102,10 +102,11 @@ func TestWalletSegments(t *testing.T) {
 	if len(got.Segments) != 1 || got.Segments[0].Source != SourceVK || !got.Segments[0].Spendable {
 		t.Errorf("vk-android wallet = %+v", got.Segments)
 	}
-	// VK iOS: only vk shown, frozen (not spendable) but the balance is visible.
+	// VK iOS: only vk shown, and spendable — the freeze is purchase-only, so VK-wallet chips still
+	// spend there (only buying more chips for money is blocked).
 	got, _ = svc.Wallet(context.Background(), id, NewContext("vk", "ios"), present)
-	if len(got.Segments) != 1 || got.Segments[0].Chips != 50 || got.Segments[0].Spendable {
-		t.Errorf("vk-ios wallet = %+v (want vk 50 frozen)", got.Segments)
+	if len(got.Segments) != 1 || got.Segments[0].Chips != 50 || !got.Segments[0].Spendable {
+		t.Errorf("vk-ios wallet = %+v (want vk 50 spendable)", got.Segments)
 	}
 }