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/recurring.yml

@@ -0,0 +1,15 @@
+# examples:
+#   periodic_cleanup:
+#     class: CleanSoftDeletedRecordsJob
+#     queue: background
+#     args: [ 1000, { batch_size: 500 } ]
+#     schedule: every hour
+#   periodic_cleanup_with_command:
+#     command: "SoftDeletedRecord.due.delete_all"
+#     priority: 2
+#     schedule: at 5am every day
+
+production:
+  clear_solid_queue_finished_jobs:
+    command: "SolidQueue::Job.clear_finished_in_batches(sleep_between_batches: 0.3)"
+    schedule: every hour at minute 12