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_mailer/invitation.html.erb

@@ -0,0 +1,4 @@
+<p>Hallo,</p>
+<p><strong><%= @inviter %></strong> hat das Dokument „<%= @title %>“ mit dir geteilt – du kannst es <%= @role %>.</p>
+<p><a href="<%= @url %>">Dokument öffnen</a></p>
+<p style="color:#6e6e73">Der Link ist 7&nbsp;Tage gültig und meldet dich gleichzeitig bei numolus an.</p>