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>

db/migrate/20260929090000_live_collaboration.rb

@@ -0,0 +1,18 @@
+# Live collaboration (Yjs): a document's content is the sum of its Yjs
+# updates. `documents.content` stays as a JSON snapshot (duplicate, export,
+# the first conversion to Yjs). The edit lock and version counter go.
+class LiveCollaboration < ActiveRecord::Migration[8.1]
+  def change
+    create_table :document_updates do |t|
+      t.references :document, type: :uuid, null: false, foreign_key: { on_delete: :cascade }
+      t.references :user, foreign_key: { on_delete: :nullify }
+      t.binary :data, null: false
+      t.datetime :created_at, null: false
+    end
+
+    remove_column :documents, :version, :integer, null: false, default: 1
+    remove_column :documents, :lock_holder, :string
+    remove_column :documents, :lock_holder_name, :string
+    remove_column :documents, :lock_expires_at, :datetime
+  end
+end