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/models.py

@@ -102,6 +102,10 @@ class User(Base, TimestampMixin, CanonicalEmailMixin):
     # current_user rejects any cookie whose epoch is stale, so a stolen/copied cookie dies on those
     # events (client-side signed cookies otherwise stay valid until they expire).
     session_epoch: Mapped[int] = mapped_column(Integer, default=0, server_default="0")
+    # When this account last signed in (Google or password), stamped by the one sign-in path,
+    # routes/auth._establish_session. NULL = not since the column existed (0085; no backfill is
+    # possible). The shared demo account is never stamped — its "last login" would mean nobody.
+    last_login_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
     display_name: Mapped[str | None] = mapped_column(String(255))
     avatar_url: Mapped[str | None] = mapped_column(String(1024))
     role: Mapped[str] = mapped_column(String(16), default="user", server_default="user")