Add Rails backend: accounts via magic link, server documents, sharing

The app becomes a Rails 8 application; the Vue frontend moves to
app/frontend and is served through vite_rails.

- Without an account documents stay in the browser (IndexedDB). Sign-in
  is by magic link or the 6-digit code from the same mail; afterwards
  local documents can be uploaded into the account.
- Server documents (/d/<id>) with versioned saves and a single-editor
  lock per browser tab; others see them read-only and follow by polling.
- Sharing via link (works without an account) and via e-mail invitation,
  roles viewer/editor; the owner manages links and people.
- Store gains a read-only mode that blocks all content changes.
- Solid Queue, Cache and Cable share the one PostgreSQL database.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

bin/bundler-audit

@@ -0,0 +1,6 @@
+#!/usr/bin/env ruby
+require_relative "../config/boot"
+require "bundler/audit/cli"
+
+ARGV.concat %w[ --config config/bundler-audit.yml ] if ARGV.empty? || ARGV.include?("check")
+Bundler::Audit::CLI.start