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/mailers/auth_mailer.rb

@@ -0,0 +1,15 @@
+class AuthMailer < ApplicationMailer
+  def magic_link(link, token, code)
+    @url = verify_auth_url(token:)
+    @code = code
+    mail to: link.email, subject: "Dein Anmeldelink für numolus"
+  end
+
+  def invitation(link, token)
+    @url = verify_auth_url(token:)
+    @inviter = link.invited_by.name
+    @title = link.document.title
+    @role = link.grants_editor? ? "bearbeiten" : "ansehen"
+    mail to: link.email, subject: "#{@inviter} hat „#{@title}“ mit dir geteilt"
+  end
+end