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/database.yml

@@ -0,0 +1,67 @@
+# PostgreSQL. Versions 9.5 and up are supported.
+#
+# Install the pg driver:
+#   gem install pg
+# On macOS with Homebrew:
+#   gem install pg -- --with-pg-config=/opt/homebrew/bin/pg_config
+# On Windows:
+#   gem install pg
+#       Choose the win32 build.
+#       Install PostgreSQL and put its /bin directory on your path.
+#
+# Configure Using Gemfile
+# gem "pg"
+#
+default: &default
+  adapter: postgresql
+  encoding: unicode
+  # For details on connection pooling, see Rails configuration guide
+  # https://guides.rubyonrails.org/configuring.html#database-pooling
+  max_connections: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
+
+
+development:
+  <<: *default
+  database: numolus_development
+
+  # The specified database role being used to connect to PostgreSQL.
+  # To create additional roles in PostgreSQL see `$ createuser --help`.
+  # When left blank, PostgreSQL will use the default role. This is
+  # the same name as the operating system user running Rails.
+  #username: numolus
+
+  # The password associated with the PostgreSQL role (username).
+  #password:
+
+  # Connect on a TCP socket. Omitted by default since the client uses a
+  # domain socket that doesn't need configuration. Windows does not have
+  # domain sockets, so uncomment these lines.
+  #host: localhost
+
+  # The TCP port the server listens on. Defaults to 5432.
+  # If your server runs on a different port number, change accordingly.
+  #port: 5432
+
+  # Schema search path. The server defaults to $user,public
+  #schema_search_path: myapp,sharedapp,public
+
+  # Minimum log levels, in increasing order:
+  #   debug5, debug4, debug3, debug2, debug1,
+  #   log, notice, warning, error, fatal, and panic
+  # Defaults to warning.
+  #min_messages: notice
+
+# Warning: The database defined as "test" will be erased and
+# re-generated from your development database when you run "rake".
+# Do not set this db to the same as development or production.
+test:
+  <<: *default
+  database: numolus_test
+
+# Production: one database for everything -- app data, Solid Queue, Solid
+# Cache and Solid Cable. The password comes from NUMOLUS_DATABASE_PASSWORD.
+production:
+  <<: *default
+  database: numolus_production
+  username: numolus
+  password: <%= ENV["NUMOLUS_DATABASE_PASSWORD"] %>