fix(auth): a cookie that lands after sign-out no longer signs the browser back in

A renewal answers with the session its request started with. Each session now
carries an id; signing out, or signing in as someone else, revokes it
(revoked_sessions, migration 0120), and a cookie carrying a revoked id is
refused. A cookie from before session ids gets one drawn from the cookie
itself. The race after log-out-everywhere fails closed and stays.

backend/app/models.py

@@ -190,6 +190,18 @@ class AuthToken(Base, TimestampMixin):
     used_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
 
 
+class RevokedSession(Base):
+    """The id of a browser session that signed out, or that a sign-in as someone else replaced
+    (`auth.revoke_session`). A request still in flight then can answer with that session's cookie,
+    re-signed by its renewal, after the sign-out cleared it; a cookie whose id is here is refused. A
+    row outlives every cookie that could carry its id, then `revoke_session` drops it."""
+
+    __tablename__ = "revoked_sessions"
+
+    sid: Mapped[str] = mapped_column(String(32), primary_key=True)
+    revoked_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), index=True)
+
+
 class Invite(Base, CanonicalEmailMixin):
     """Access-request / whitelist row. Source of truth for who may sign in; the env
     ALLOWED_EMAILS/ADMIN_EMAILS act only as a bootstrap fallback (see auth.is_allowed)."""