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/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.1].define(version: 2026_09_28_120300) do
+ActiveRecord::Schema[8.1].define(version: 2026_09_29_090000) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "citext"
   enable_extension "pg_catalog.plpgsql"
@@ -31,16 +31,21 @@ ActiveRecord::Schema[8.1].define(version: 2026_09_28_120300) do
     t.index ["user_id"], name: "index_document_memberships_on_user_id"
   end
 
+  create_table "document_updates", force: :cascade do |t|
+    t.uuid "document_id", null: false
+    t.bigint "user_id"
+    t.binary "data", null: false
+    t.datetime "created_at", null: false
+    t.index ["document_id"], name: "index_document_updates_on_document_id"
+    t.index ["user_id"], name: "index_document_updates_on_user_id"
+  end
+
   create_table "documents", id: :uuid, default: -> { "gen_random_uuid()" }, force: :cascade do |t|
     t.bigint "owner_id", null: false
     t.string "title", null: false
     t.text "content", null: false
-    t.integer "version", default: 1, null: false
     t.bigint "updated_by_id"
     t.datetime "content_updated_at", null: false
-    t.string "lock_holder"
-    t.string "lock_holder_name"
-    t.datetime "lock_expires_at"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
     t.index ["owner_id"], name: "index_documents_on_owner_id"
@@ -273,6 +278,8 @@ ActiveRecord::Schema[8.1].define(version: 2026_09_28_120300) do
   add_foreign_key "document_memberships", "documents", on_delete: :cascade
   add_foreign_key "document_memberships", "share_links", on_delete: :cascade
   add_foreign_key "document_memberships", "users", on_delete: :cascade
+  add_foreign_key "document_updates", "documents", on_delete: :cascade
+  add_foreign_key "document_updates", "users", on_delete: :nullify
   add_foreign_key "documents", "users", column: "owner_id", on_delete: :cascade
   add_foreign_key "documents", "users", column: "updated_by_id", on_delete: :nullify
   add_foreign_key "magic_links", "documents", on_delete: :cascade