Harden S3 auth boundaries

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

auth/policy.go

@@ -33,14 +33,16 @@ func statementMatches(stmt models.AuthPolicyStatement, target RequestTarget) boo
 	if !bucketMatches(stmt.Bucket, target.Bucket) {
 		return false
 	}
-	if target.Key == "" {
-		return true
-	}
-
 	prefix := strings.TrimSpace(stmt.Prefix)
 	if prefix == "" || prefix == "*" {
 		return true
 	}
+	if target.Key == "" {
+		if target.Action == ActionListBucket {
+			return strings.HasPrefix(target.Prefix, prefix)
+		}
+		return true
+	}
 	return strings.HasPrefix(target.Key, prefix)
 }