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>

config/routes.rb

@@ -0,0 +1,33 @@
+Rails.application.routes.draw do
+  get "up" => "rails/health#show", as: :rails_health_check
+
+  # sign-in / invitation link from the mail: confirm page, then POST signs in
+  get "auth/verify" => "auth#show", as: :verify_auth
+  post "auth/verify" => "auth#create"
+
+  namespace :api do
+    post "auth/magic_link" => "auth#magic_link"
+    post "auth/code" => "auth#code"
+    delete "auth/session" => "auth#logout"
+
+    get "me" => "me#show"
+    patch "me" => "me#update"
+
+    resources :documents, only: %i[index create show update destroy] do
+      resource :lock, only: %i[create destroy], controller: "document_locks"
+      resource :join, only: :create
+      get "share" => "shares#show"
+      post "share/links" => "shares#create_link"
+      delete "share/links/:id" => "shares#revoke_link"
+      post "share/invitations" => "shares#invite"
+      patch "share/members/:user_id" => "shares#update_member"
+      delete "share/members/:user_id" => "shares#remove_member"
+    end
+  end
+
+  mount LetterOpenerWeb::Engine, at: "/letter_opener" if Rails.env.development?
+
+  # the Vue app
+  get "d/:id" => "spa#show", as: :document_page
+  root "spa#show"
+end