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>

app/controllers/auth_controller.rb

@@ -0,0 +1,22 @@
+# The page behind the link in a sign-in / invitation mail. Opening the link
+# (GET) only shows a button; the sign-in happens on POST. Mail scanners
+# fetch links on their own and would otherwise use up the one-time token.
+class AuthController < ApplicationController
+  layout "auth"
+  rate_limit to: 30, within: 15.minutes, only: :create
+
+  def show
+    @token = params[:token].to_s
+    @link = MagicLink.find_by_token(@token)
+    render :invalid, status: :not_found unless @link
+  end
+
+  def create
+    link = MagicLink.find_by_token(params[:token])
+    return render(:invalid, status: :not_found) unless link
+    start_session_for(link.redeem!)
+    redirect_to link.destination
+  rescue ActiveRecord::RecordNotFound
+    render :invalid, status: :not_found
+  end
+end