Reset former members' connections and grant open rooms in one statement

Deleting the memberships with delete_all skips Membership's after_destroy_commit, which resets a member's
connections when one membership is revoked. Until the job ran, a member who had the room open kept its
streams, and a message that still landed in the room (a request already past the membership check, a bot's
reply) reached them. The request now reads the members' ids in the transaction that deletes their
memberships, and the job resets their connections before it destroys the messages. It costs a Redis round
trip per member, about 1.5 s for 10,000, so it's done in the job rather than in the request.

User#grant_membership_to_open_rooms read the open rooms and inserted in a separate statement, so a user
created while a room was being closed could read it as open and be granted it after the close. It's now a
single insert ... select, which SQLite runs under the write lock, skipping duplicates as insert_all did.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bj8KnxpTf9sj2Ysa8aLAVa

test/models/user_test.rb

@@ -12,6 +12,18 @@ class UserTest < ActiveSupport::TestCase
     end
   end
 
+  test "memberships granted to the open rooms are like any other" do
+    freeze_time
+    user = create_new_user
+
+    assert_equal Rooms::Open.ids.sort, user.memberships.pluck(:room_id).sort
+    user.memberships.each do |membership|
+      assert membership.involved_in_mentions?
+      assert_equal Time.current, membership.created_at
+      assert_equal Time.current, membership.updated_at
+    end
+  end
+
   test "deactivating a user deletes push subscriptions, searches, memberships for non-direct rooms, and changes their email address" do
     assert_difference -> { Membership.count }, -users(:david).memberships.without_direct_rooms.count do
     assert_difference -> { Push::Subscription.count }, -users(:david).push_subscriptions.count do