feat(09-01): reject cross-audience tokens on both guards

- Frontend verification accepts a missing audience for PHP tokens
- An explicit audience must match the guard, even when the secret is shared

bouncer/jwt.go

@@ -153,23 +153,11 @@ func (g *jwtGuard) WriteUnauthorized(w http.ResponseWriter, err error) {
 }
 
 // Verify parses a token with HS256 pinned and a required exp and sub.
+// A missing audience is accepted for PHP-issued frontend tokens. An explicit
+// audience must be AudienceUser, so a backend token cannot pass this guard.
 func Verify(tokenString, secret string) (string, error) {
-	if strings.TrimSpace(secret) == "" {
-		return "", fmt.Errorf("bouncer: jwt secret is empty")
-	}
-	parser := jwt.NewParser(jwt.WithValidMethods([]string{"HS256"}), jwt.WithExpirationRequired())
-	claims := jwt.MapClaims{}
-	_, err := parser.ParseWithClaims(tokenString, claims, func(t *jwt.Token) (any, error) {
-		return []byte(secret), nil
-	})
-	if err != nil {
-		return "", mapJWTError(err)
-	}
-	sub := subject(claims)
-	if sub == "" {
-		return "", errors.New(msgRequiredClaims)
-	}
-	return sub, nil
+	sub, _, _, _, err := verifyClaims(tokenString, secret, "")
+	return sub, err
 }
 
 // VerifyClaims parses a token the same way Verify does and also returns iat, exp, and jti.
@@ -198,7 +186,7 @@ func verifyClaims(tokenString, secret, audience string) (sub string, iat, exp ti
 	if err != nil {
 		return "", time.Time{}, time.Time{}, "", mapJWTError(err)
 	}
-	if audience != "" && !audienceMatches(claims, audience) {
+	if !frontendAudienceOK(claims, audience) {
 		return "", time.Time{}, time.Time{}, "", errors.New(msgBadSignature)
 	}
 	sub = subject(claims)
@@ -211,6 +199,16 @@ func verifyClaims(tokenString, secret, audience string) (sub string, iat, exp ti
 	return sub, iat, exp, jti, nil
 }
 
+// frontendAudienceOK accepts a missing audience when expected is empty
+// (legacy frontend tokens) and otherwise requires expected.
+func frontendAudienceOK(claims jwt.MapClaims, expected string) bool {
+	auds := claimAudiences(claims)
+	if expected == "" {
+		return len(auds) == 0 || audienceMatches(claims, AudienceUser)
+	}
+	return audienceMatches(claims, expected)
+}
+
 func audienceMatches(claims jwt.MapClaims, expected string) bool {
 	for _, aud := range claimAudiences(claims) {
 		if aud == expected {