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/service_intake_test.go

@@ -28,7 +28,7 @@ func TestCreateOrderGateRejections(t *testing.T) {
 		cxt  Context
 	}{
 		{"untrusted context", Context{}},
-		{"vk-ios spend freeze", Context{Kind: SourceVK, Subtype: SubtypeIOS}},
+		{"vk-ios purchase freeze", Context{Kind: SourceVK, Subtype: SubtypeIOS}},
 		{"method segment not attached", Context{Kind: SourceTelegram}},
 	}
 	for _, tc := range cases {