fix(users): a password reset signs out every session begun with the old password (#5427)

#5427 "A password reset doesn't end the sessions already signed in with
the old password": users.sessions_since (migration 0038) is set on reset;
refresh_session_user ends a session whose login_at is earlier. An admin
resetting their own password carries on signed in.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/migrations/versions/0038_user_sessions_since.py

@@ -0,0 +1,23 @@
+"""A password reset ends the account's other sessions (#5427): sessions that
+began before users.sessions_since are over.
+
+Revision ID: 0038_user_sessions_since
+Revises: 0037_user_invites
+Create Date: 2026-10-08
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0038_user_sessions_since"
+down_revision: Union[str, None] = "0037_user_invites"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("users", sa.Column("sessions_since", sa.DateTime(timezone=True), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("users", "sessions_since")