Live collaboration with Yjs over ActionCable

All editors now work on a server document at the same time instead of
taking turns behind an edit lock.

- The document's content is a sequence of Yjs updates (document_updates);
  DocumentChannel stores and relays them immediately, checks access on
  subscribe and again whenever links or memberships change.
- app/frontend/collab/: the store stays position-based; a binding maps it
  to a Yjs form with stable row/column ids, so a row inserted by one
  person doesn't misplace what another is typing. Remote changes are
  applied only after pending local ones went into Yjs.
- Per-person undo (Y.UndoManager), presence via awareness: the others'
  selections as coloured boxes with names, avatars in the toolbar.
- Existing JSON documents convert on first open, deterministically, so
  two people opening one at once end up with the same Yjs state.
- Editors compact the stored updates after 200 and keep a JSON snapshot
  current for export and duplicating. Edit lock and polling are gone.

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

test/models/document_test.rb

@@ -49,15 +49,4 @@ class DocumentTest < ActiveSupport::TestCase
     DocumentMembership.grant!(document: @doc, user: bob, role: :viewer, source: :link)
     assert_equal :editor, @doc.role_for(user: bob)
   end
-
-  test "edit lock: one holder at a time, expires without heartbeat" do
-    assert @doc.acquire_lock!("1:tab-a", "Anna")
-    assert @doc.acquire_lock!("1:tab-a", "Anna"), "refreshing own lock"
-    assert_not @doc.acquire_lock!("2:tab-b", "Bob")
-    assert @doc.reload.locked_by_other?("2:tab-b")
-
-    travel Document::LOCK_TTL + 1.second do
-      assert @doc.acquire_lock!("2:tab-b", "Bob")
-    end
-  end
 end