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>

eslint.config.js

@@ -4,7 +4,7 @@ import skipFormatting from '@vue/eslint-config-prettier/skip-formatting';
 
 export default [
   {
-    ignores: ['dist/**', 'node_modules/**', 'coverage/**'],
+    ignores: ['dist/**', 'node_modules/**', 'coverage/**', 'public/**', 'vendor/**', 'tmp/**', 'log/**'],
   },
   ...pluginVue.configs['flat/recommended'],
   ...vueTsEslintConfig(),