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/controllers/api/shares_controller.rb

@@ -18,12 +18,14 @@ module Api
         @doc.share_links.active.where(role:).find_each(&:revoke!)
         @doc.share_links.create!(role:, created_by: Current.user)
       end
+      @doc.revalidate_access!
       render json: share_json
     end
 
     # DELETE /api/documents/:document_id/share/links/:id
     def revoke_link
       @doc.share_links.active.find(params[:id]).revoke!
+      @doc.revalidate_access!
       render json: share_json
     end
 
@@ -44,12 +46,14 @@ module Api
     def update_member
       m = @doc.document_memberships.find_by!(user_id: params[:user_id])
       m.update!(role: params.require(:role))
+      @doc.revalidate_access!
       render json: share_json
     end
 
     # DELETE /api/documents/:document_id/share/members/:user_id
     def remove_member
       @doc.document_memberships.where(user_id: params[:user_id]).delete_all
+      @doc.revalidate_access!
       render json: share_json
     end