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>

tsconfig.json

@@ -21,6 +21,6 @@
 
     "types": ["vitest/globals"]
   },
-  "include": ["src/**/*.ts", "src/**/*.d.ts", "src/**/*.vue", "vite.config.ts"],
+  "include": ["app/frontend/**/*.ts", "app/frontend/**/*.d.ts", "app/frontend/**/*.vue", "vite.config.ts"],
   "exclude": ["node_modules", "dist"]
 }