Fix UserInfo canonical subject resolution

Assistant: codex
Assistant-Model: gpt-5.6-sol
Assistant-Session: 01a058f3-8ba0-7692-a042-9a870fc3d663

src/internal/server/oidc/userinfo.go

@@ -1,6 +1,7 @@
 package oidc
 
 import (
+	"context"
 	"crypto"
 	"crypto/rsa"
 	"crypto/sha256"
@@ -52,8 +53,11 @@ func (h *UserinfoHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// 4. Look up user by sub (sub IS the username per spec §3.1).
-	user, err := h.Users.LookupUser(ctx, sub)
+	// 4. Resolve the canonical subject. Human tokens use the stable directory
+	// ID (the LDAP DN) as sub, while LookupUser accepts an LDAP uid. Prefer the
+	// scoped username hint but require it to resolve back to the signed subject;
+	// fall back to an ID scan for openid-only tokens and renamed users.
+	user, err := lookupUserBySubject(ctx, h.Users, claims, sub)
 	if err != nil {
 		// User referenced in token but not found → treat as invalid token.
 		http.Error(w, `{"error":"invalid_token","description":"subject not found"}`, http.StatusUnauthorized)
@@ -97,6 +101,36 @@ func (h *UserinfoHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	_ = json.NewEncoder(w).Encode(resp)
 }
 
+func lookupUserBySubject(
+	ctx context.Context,
+	users domain.UserRepository,
+	claims map[string]interface{},
+	sub string,
+) (*domain.User, error) {
+	if username, _ := claims["preferred_username"].(string); username != "" {
+		if user, err := users.LookupUser(ctx, username); err == nil && user.ID == sub {
+			return user, nil
+		}
+	}
+
+	// Retain compatibility with older signed tokens whose subject was the uid.
+	if user, err := users.LookupUser(ctx, sub); err == nil &&
+		(user.ID == sub || user.Username == sub) {
+		return user, nil
+	}
+
+	all, err := users.ListUsers(ctx)
+	if err != nil {
+		return nil, err
+	}
+	for i := range all {
+		if all[i].ID == sub {
+			return &all[i], nil
+		}
+	}
+	return nil, domain.ErrUserNotFound
+}
+
 // ---------------------------------------------------------------------------
 // JWT validation (stdlib only — no external JWT library)
 // ---------------------------------------------------------------------------