fix(auth): reject invalid bearer tokens instead of falling back

Modify the authentication handler to return an unauthorized error when
an invalid or disabled bearer token is provided, rather than silently
falling back to an anonymous request.

This ensures that clients attempting to authenticate but failing (due to
expired, malformed, or disabled tokens) are explicitly notified of the
auth failure instead of proceeding anonymously. True anonymous requests
without any Authorization header remain supported.

backend/libs/handlers/auth.go

@@ -251,23 +251,32 @@ func (a *App) loginAndRedirect(w http.ResponseWriter, r *http.Request, email, pa
 }
 
 func (a *App) currentUser(r *http.Request) (services.User, bool) {
+	user, ok, _ := a.currentUserWithAuthError(r)
+	return user, ok
+}
+
+func (a *App) currentUserWithAuthError(r *http.Request) (services.User, bool, error) {
 	// Personal access tokens via Authorization: Bearer act as their owning user.
 	// A bearer header is never set by browsers cross-site, so this path is not
 	// subject to CSRF and intentionally bypasses the session cookie.
 	if header := r.Header.Get("Authorization"); header != "" {
 		if raw, ok := strings.CutPrefix(header, "Bearer "); ok {
-			if user, err := a.authService.UserForAPIToken(raw); err == nil {
-				return user, true
+			user, err := a.authService.UserForAPIToken(raw)
+			if err != nil {
+				return services.User{}, false, err
 			}
-			return services.User{}, false
+			return user, true, nil
 		}
 	}
 	cookie, err := r.Cookie(userSessionCookieName)
 	if err != nil {
-		return services.User{}, false
+		return services.User{}, false, nil
 	}
 	user, _, err := a.authService.UserForSession(cookie.Value)
-	return user, err == nil
+	if err != nil {
+		return services.User{}, false, nil
+	}
+	return user, true, nil
 }
 
 func (a *App) requireUser(w http.ResponseWriter, r *http.Request) (services.User, bool) {