Refuse a session for a password that changed after it was checked

A password change ends every session, but a sign-in or sign-in link checked
against the old password just before the change could still save its session
just after, and that session outlived the change.

The session now checks, as it's saved, that the user's password is still the
one the request loaded. The save's transaction holds SQLite's write lock from
the start, so a change commits either before the check, which fails it, or
after the insert, and ends this session with the others. Sessions start
through the user's association so the check sees the user that was
authenticated, and a refused sign-in is answered like a wrong password.

test/controllers/sessions/transfers_controller_test.rb

@@ -30,4 +30,19 @@ class Sessions::TransfersControllerTest < ActionDispatch::IntegrationTest
     assert_response :bad_request
     assert_nil parsed_cookies.signed[:session_token]
   end
+
+  test "update is refused when the password changes after the link was checked" do
+    user = users(:david)
+    transfer_id = user.transfer_id
+    stale = User.find(user.id)
+    User.find(user.id).update!(password: "a new secret123456")
+    User.stubs(:find_by_transfer_id).with(transfer_id).returns(stale)
+
+    assert_no_difference -> { Session.count } do
+      put session_transfer_url(transfer_id)
+    end
+
+    assert_response :bad_request
+    assert_nil parsed_cookies.signed[:session_token]
+  end
 end