Linked email identity `last_sign_in_at` stays stale after password sign-in

# Bug report - [x] I confirm this is a bug with Supabase, not with my own application. - [x] I confirm I have searched the [Docs](https://docs.supabase.com), GitHub [Discussions](https://github.com/supabase/supabase/discussions), and [Discord](https://discord.supabase.com). ## Describe the bug For a user with linked `email` and `facebook` identities, signing in with email/password updates `user.last_sign_in_at` but does not update the `email` identity's `last_sign_in_at`. This makes the auth payload internally inconsistent and breaks any attempt to infer the current session provider from `user.identities[].last_sign_in_at`. ## To Reproduce 1. Use a Supabase Auth user with linked `email` and `facebook` identities. 2. Sign in with email/password using `POST /auth/v1/token?grant_type=password`. 3. Inspect the returned `user` payload. 4. Inspect the same user through `GET /auth/v1/admin/users/{user_id}`. ## Expected behavior After a successful password sign-in, both of these should reflect the sign-in event: - `user.last_sign_in_at` - the `email` identity's `last_sign_in_at` ## Screenshots No screenshots included. The issue is reproduced directly from the auth payload and admin user record. ## System information - OS: macOS - Browser (if applies): Chrome - Version of supabase-js: `^2.79.0` - Version of Node.js: `22.19.0` ## Additional context I reproduced this live on March 20, 2026. A fresh email/password sign-in updated the user-level timestamp but left the `email` identity stale. Observed values after a fresh password sign-in: - `user.last_sign_in_at`: `2026-03-20T02:17:30.723417Z` - `email.last_sign_in_at`: `2025-11-05T22:12:50.443863Z` - `facebook.last_sign_in_at`: `2026-03-06T22:23:46.76262Z` - `app_metadata.provider`: `email` - `app_metadata.providers`: `["email", "facebook"]` I also confirmed the same inconsistency is returned directly from the password sign-in response itself, not just from the admin API. A later direct sign-in response on March 20, 2026 return…