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.rb

@@ -1,6 +1,5 @@
 class Document < ApplicationRecord
   ROLES = %w[viewer editor owner].freeze
-  LOCK_TTL = 90.seconds
   MAX_CONTENT_BYTES = 20.megabytes
 
   belongs_to :owner, class_name: "User"
@@ -8,6 +7,7 @@ class Document < ApplicationRecord
   has_many :document_memberships, dependent: :delete_all
   has_many :members, through: :document_memberships, source: :user
   has_many :share_links, dependent: :delete_all
+  has_many :updates, class_name: "DocumentUpdate", dependent: :delete_all
 
   validates :title, presence: true, length: { maximum: 200 }
   validates :content, presence: true
@@ -36,39 +36,28 @@ class Document < ApplicationRecord
     role.present? && ROLES.index(role.to_s) >= ROLES.index(minimum.to_s)
   end
 
-  # ---- edit lock --------------------------------------------------------
-  def locked_by_other?(holder)
-    lock_holder.present? && lock_holder != holder && lock_expires_at&.future?
-  end
-
-  # Take or refresh the lock for `holder`. Returns true when `holder` has it.
-  def acquire_lock!(holder, name)
-    with_lock do
-      return false if locked_by_other?(holder)
-      update_columns(lock_holder: holder, lock_holder_name: name, lock_expires_at: LOCK_TTL.from_now)
-      true
-    end
-  end
-
-  def release_lock!(holder)
-    with_lock do
-      update_columns(lock_holder: nil, lock_holder_name: nil, lock_expires_at: nil) if lock_holder == holder
-    end
-  end
-
-  def lock_info(holder)
-    { locked_by_other: locked_by_other?(holder), holder_name: (lock_holder_name if locked_by_other?(holder)) }
-  end
-
   def meta_json(role:)
     {
-      id:, title:, version:, role:,
+      id:, title:, role:,
       owner_name: owner.name,
       updated_at: content_updated_at.iso8601(3),
       updated_by_name: updated_by&.name
     }
   end
 
+  # Tell everyone connected to this document (see DocumentChannel).
+  def broadcast(message) = DocumentChannel.broadcast_to(self, message)
+
+  # Access changed: every connection checks its role again.
+  def revalidate_access! = broadcast({ type: "revalidate" })
+
+  def self.valid_snapshot?(json)
+    parsed = JSON.parse(json)
+    parsed.is_a?(Hash) && parsed["sheets"].is_a?(Array)
+  rescue JSON::ParserError, TypeError
+    false
+  end
+
   private
 
   def content_is_document_json