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>

app/models/document_membership.rb

@@ -0,0 +1,25 @@
+class DocumentMembership < ApplicationRecord
+  belongs_to :document
+  belongs_to :user
+  belongs_to :share_link, optional: true
+
+  enum :role, { viewer: 0, editor: 1 }, validate: true
+  enum :source, { invite: 0, link: 1 }, validate: true
+
+  # Grant access. An invitation is explicit: it sets exactly the invited role
+  # and survives revoking any link. A link never weakens existing access and
+  # never turns an invitation back into link-based access.
+  def self.grant!(document:, user:, role:, source:, share_link: nil)
+    return if document.owner_id == user.id
+    m = find_or_initialize_by(document:, user:)
+    if source.to_s == "invite"
+      m.assign_attributes(role:, source: :invite, share_link: nil)
+    elsif m.new_record?
+      m.assign_attributes(role:, source: :link, share_link:)
+    elsif m.link? && roles[role.to_s] > roles[m.role]
+      m.assign_attributes(role:, share_link:)
+    end
+    m.save!
+    m
+  end
+end