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>

app/models/document_update.rb

@@ -0,0 +1,13 @@
+# One Yjs update of a document -- an edit by someone, or a compacted state
+# that replaces many earlier updates. Order doesn't matter to Yjs.
+class DocumentUpdate < ApplicationRecord
+  MAX_BYTES = 10.megabytes
+
+  belongs_to :document
+  belongs_to :user, optional: true
+
+  validates :data, presence: true
+  validate { errors.add(:data, "ist zu groß") if data && data.bytesize > MAX_BYTES }
+
+  def as_message = { id:, data: Base64.strict_encode64(data) }
+end