fix(auth): refusing a revoked session no longer deletes a newer sign-in's cookie

The refusal cleared the session, so a request in flight with the previous account's cookie
answered with a cookie deletion that could land after a sign-in as someone else. A revoked id is
refused on every request anyway. Also a test for the pruning of old revocations.

backend/tests/test_auth_session.py

@@ -205,7 +205,10 @@ def test_a_cookie_from_before_a_sign_in_as_another_account_is_refused(client, ma
     assert _login(client, "late2b@example.com", _PW).status_code == 200
     bob = client.cookies.get("session")
     _hold(client, in_flight)
-    assert client.get(_PROTECTED).status_code == 401
+    r = client.get(_PROTECTED)
+    assert r.status_code == 401
+    # The refusal deletes no cookie: landing after Bob's, a deletion would sign Bob out.
+    assert "set-cookie" not in r.headers
     _hold(client, bob)  # Bob's own session is untouched
     assert client.get("/api/me").json()["email"] == "late2b@example.com"
 
@@ -225,6 +228,28 @@ def test_a_session_from_before_session_ids_gets_one_and_its_sign_out_holds(clien
     assert client.get(_PROTECTED).status_code == 401
 
 
+def test_a_revoke_drops_only_the_revocations_no_cookie_can_outlive(db):
+    # Each revoke prunes the table: a row goes once no cookie carrying its id can still be valid, and
+    # never sooner — a later sign-out must not undo an earlier one.
+    from datetime import datetime, timezone
+    from types import SimpleNamespace
+
+    from sqlalchemy import select
+
+    from app.auth import SESSION_MAX_AGE, revoke_session
+    from app.models import RevokedSession
+
+    now = datetime.now(timezone.utc)
+    life = timedelta(seconds=SESSION_MAX_AGE)
+    db.add(RevokedSession(sid="outlived", revoked_at=now - life - timedelta(days=2)))
+    db.add(RevokedSession(sid="still-live", revoked_at=now - life + timedelta(hours=1)))
+    db.commit()
+    revoke_session(db, SimpleNamespace(session={"user_id": 1, "sid": "this-one"}, cookies={}))
+    db.commit()
+    left = set(db.scalars(select(RevokedSession.sid)).all())
+    assert left == {"still-live", "this-one"}
+
+
 def _session(client) -> dict:
     """The decoded payload of the client's session cookie."""
     return _decode(client.cookies.get("session"))