CRITICAL: Add audience validation for access tokens

Access tokens include an 'aud' (audience) claim set to the client ID,
but this was never validated during token validation. This allowed
tokens issued for one client to be used by another client, violating
the OAuth 2.0 security model.

Changes:
- Add ValidateAccessTokenForClient method that validates audience
  if expectedClientID is provided
- Update ValidateAccessToken to call ValidateAccessTokenForClient
  (backward compatible, no audience check if not specified)
- Update userinfo endpoint to accept optional client_id parameter
  and validate token audience matches it

Security impact:
- Prevents token reuse across different clients
- Ensures tokens are scoped to specific clients as intended
- Prevents attackers from using tokens issued for one client to
  access resources protected by another client

internal/service/oidc_service.go

@@ -13,6 +13,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math/big"
 	"os"
 	"strings"
 	"time"
@@ -538,6 +539,13 @@ func (oidc *OIDCService) GenerateAccessToken(userContext *config.UserContext, cl
 }
 
 func (oidc *OIDCService) ValidateAccessToken(accessToken string) (*config.UserContext, error) {
+	return oidc.ValidateAccessTokenForClient(accessToken, "")
+}
+
+// ValidateAccessTokenForClient validates an access token and optionally checks the audience claim.
+// If expectedClientID is provided, validates that the token's audience matches the expected client ID.
+// This prevents tokens issued for one client from being used by another client.
+func (oidc *OIDCService) ValidateAccessTokenForClient(accessToken string, expectedClientID string) (*config.UserContext, error) {
 	token, err := jwt.Parse(accessToken, func(token *jwt.Token) (interface{}, error) {
 		if _, ok := token.Method.(*jwt.SigningMethodRSA); !ok {
 			return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
@@ -564,6 +572,14 @@ func (oidc *OIDCService) ValidateAccessToken(accessToken string) (*config.UserCo
 		return nil, errors.New("invalid issuer")
 	}
 
+	// Verify audience if expected client ID is provided
+	if expectedClientID != "" {
+		aud, ok := claims["aud"].(string)
+		if !ok || aud != expectedClientID {
+			return nil, errors.New("invalid audience")
+		}
+	}
+
 	// Check expiration
 	exp, ok := claims["exp"].(float64)
 	if !ok || time.Now().Unix() > int64(exp) {
@@ -629,11 +645,8 @@ func (oidc *OIDCService) GetJWKS() (map[string]interface{}, error) {
 	e := oidc.publicKey.E
 
 	nBytes := n.Bytes()
-	eBytes := make([]byte, 4)
-	eBytes[0] = byte(e >> 24)
-	eBytes[1] = byte(e >> 16)
-	eBytes[2] = byte(e >> 8)
-	eBytes[3] = byte(e)
+	// Use minimal-octet encoding for exponent per RFC 7517
+	eBytes := big.NewInt(int64(e)).Bytes()
 
 	jwk := map[string]interface{}{
 		"kty": "RSA",