Add OAuth2 API scope registration and enforcement

Register v1 API scopes in coredata, advertise them in OIDC discovery
and protected-resource metadata, show them on the consent screen, and
enforce scope-to-action mapping in the IAM Authorizer before policy
evaluation.

Signed-off-by: Ludovic Vielle <ludovic@probo.com>

pkg/iam/oauth2/pkce.go

@@ -0,0 +1,43 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package oauth2
+
+import (
+	"crypto/sha256"
+	"crypto/subtle"
+	"encoding/base64"
+
+	"go.probo.inc/probo/pkg/coredata"
+)
+
+func ValidateCodeChallenge(verifier, challenge string, method coredata.OAuth2CodeChallengeMethod) bool {
+	if verifier == "" || challenge == "" {
+		return false
+	}
+
+	switch method {
+	case coredata.OAuth2CodeChallengeMethodS256:
+		return validateS256(verifier, challenge)
+	default:
+		return false
+	}
+}
+
+func validateS256(verifier, challenge string) bool {
+	h := sha256.Sum256([]byte(verifier))
+	computed := base64.RawURLEncoding.EncodeToString(h[:])
+
+	return subtle.ConstantTimeCompare([]byte(computed), []byte(challenge)) == 1
+}