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/integration/documents_api_test.rb

@@ -1,14 +1,16 @@
 require "test_helper"
 
 class DocumentsApiTest < ActionDispatch::IntegrationTest
+  include ActionCable::TestHelper
+
   setup do
     @anna = create_user("anna@example.com")
     @bob = create_user("bob@example.com")
     @doc = create_document(@anna)
   end
 
-  def put_content(doc, base_version, headers: {})
-    patch "/api/documents/#{doc.id}", params: { content: DOC_JSON, base_version: }, headers:, as: :json
+  def rename(doc, title, headers: {})
+    patch "/api/documents/#{doc.id}", params: { title: }, headers:, as: :json
   end
 
   test "listing and creating need an account" do
@@ -38,39 +40,6 @@ class DocumentsApiTest < ActionDispatch::IntegrationTest
     assert_response :not_found
   end
 
-  test "save with version check; stale base_version is a conflict" do
-    sign_in @anna
-    put_content(@doc, 1)
-    assert_response :success
-    assert_equal 2, json.dig("document", "version")
-
-    put_content(@doc, 1)
-    assert_response :conflict
-    assert_equal 2, json["version"]
-  end
-
-  test "known_version returns metadata only when unchanged" do
-    sign_in @anna
-    get "/api/documents/#{@doc.id}", params: { known_version: 1 }
-    assert json["unchanged"]
-    assert_nil json.dig("document", "content")
-  end
-
-  test "edit lock blocks saves from another tab" do
-    sign_in @anna
-    post "/api/documents/#{@doc.id}/lock", headers: { "X-Client-Id" => "tab-a" }
-    assert json.dig("lock", "mine")
-
-    post "/api/documents/#{@doc.id}/lock", headers: { "X-Client-Id" => "tab-b" }
-    assert_not json.dig("lock", "mine")
-    put_content(@doc, 1, headers: { "X-Client-Id" => "tab-b" })
-    assert_response :locked
-
-    delete "/api/documents/#{@doc.id}/lock", headers: { "X-Client-Id" => "tab-a" }
-    put_content(@doc, 1, headers: { "X-Client-Id" => "tab-b" })
-    assert_response :success
-  end
-
   test "share link without account: read with token; viewer cannot write" do
     link = @doc.share_links.create!(role: :viewer, created_by: @anna)
     headers = { "X-Share-Token" => link.token }
@@ -79,7 +48,7 @@ class DocumentsApiTest < ActionDispatch::IntegrationTest
     assert_equal "viewer", json.dig("document", "role")
     get "/api/documents/#{@doc.id}", headers: headers
     assert_response :success
-    put_content(@doc, 1, headers:)
+    rename(@doc, "Neu", headers:)
     assert_response :forbidden
 
     get "/api/documents/#{@doc.id}", headers: { "X-Share-Token" => "wrong" }
@@ -92,10 +61,18 @@ class DocumentsApiTest < ActionDispatch::IntegrationTest
     post "/api/documents/#{@doc.id}/join", params: { token: link.token }, as: :json
     get "/api/documents"
     assert_equal [ [ @doc.id, "editor" ] ], json["documents"].map { [ _1["id"], _1["role"] ] }
-    put_content(@doc, 1)
+    rename(@doc, "Von Bob")
     assert_response :success
   end
 
+  test "renaming tells everyone connected" do
+    sign_in @anna
+    assert_broadcast_on(DocumentChannel.broadcasting_for(@doc), { type: "meta", title: "Neu" }) do
+      rename(@doc, "Neu")
+    end
+    assert_equal "Neu", @doc.reload.title
+  end
+
   test "owner deletes; a member only leaves" do
     DocumentMembership.grant!(document: @doc, user: @bob, role: :editor, source: :invite)
     sign_in @bob