feat(cap): expose CaptchaService and unversioned /api/cap routes

backend/core/contracts/captcha.go

@@ -0,0 +1,12 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package contracts
+
+// CaptchaService defines the contract for CAPTCHA challenge issuance,
+// redemption, and scoped verification middleware.
+type CaptchaService interface {
+	VerifyMiddleware(scope string) any
+	ChallengeHandler() any
+	RedeemHandler() any
+}