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/views/auth/invalid.html.erb

@@ -0,0 +1,3 @@
+<h1>Link nicht mehr gültig</h1>
+<p>Dieser Link wurde schon verwendet oder ist abgelaufen. Fordere in numolus einfach einen neuen an.</p>
+<a class="btn" href="/">Zu numolus</a>

app/views/auth/show.html.erb

@@ -0,0 +1,11 @@
+<% if @link.invitation? %>
+  <h1>Einladung annehmen</h1>
+  <p><strong><%= @link.invited_by.name %></strong> hat dich zu „<%= @link.document.title %>“ eingeladen. Du wirst dabei als <%= @link.email %> angemeldet.</p>
+<% else %>
+  <h1>Bei numolus anmelden</h1>
+  <p>Du meldest dich als <strong><%= @link.email %></strong> an.</p>
+<% end %>
+<%= form_with url: verify_auth_path, method: :post do %>
+  <%= hidden_field_tag :token, @token %>
+  <button type="submit"><%= @link.invitation? ? "Annehmen und öffnen" : "Anmelden" %></button>
+<% end %>