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>

test/test_helper.rb

@@ -0,0 +1,32 @@
+ENV["RAILS_ENV"] ||= "test"
+require_relative "../config/environment"
+require "rails/test_help"
+
+module ActiveSupport
+  class TestCase
+    # Run tests in parallel with specified workers
+    parallelize(workers: :number_of_processors)
+
+    DOC_JSON = { version: 2, sheets: [ { id: "s1", name: "Blatt 1", objects: [] } ] }.to_json
+
+    def create_user(email = "anna@example.com") = User.create!(email:)
+
+    def create_document(owner, title: "Budget") =
+      owner.owned_documents.create!(title:, content: DOC_JSON)
+  end
+end
+
+module SignInHelper
+  # signs in through the real code flow
+  def sign_in(user)
+    link = MagicLink.issue!(email: user.email)
+    post "/api/auth/code", params: { email: user.email, code: link.code }, as: :json
+    assert_response :success
+  end
+
+  def json = response.parsed_body
+end
+
+class ActionDispatch::IntegrationTest
+  include SignInHelper
+end