feat: record when an account last signed in

users.last_login_at (migration 0085, no backfill) is stamped by _establish_session,
which the Google callback now shares with password login instead of repeating its
session setup inline. The demo entry does not stamp its shared account. The admin
user list carries the field.

backend/app/routes/admin.py

@@ -408,6 +408,8 @@ def _serialize_user(u: User) -> dict:
         # confusing sign-in.
         "google_email": u.google_email if u.google_email != u.email else None,
         "created_at": iso(u.created_at),
+        # NULL = no sign-in since the column existed (0085), or the shared demo account.
+        "last_login_at": iso(u.last_login_at),
     }