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.rb

@@ -0,0 +1,84 @@
+class Document < ApplicationRecord
+  ROLES = %w[viewer editor owner].freeze
+  LOCK_TTL = 90.seconds
+  MAX_CONTENT_BYTES = 20.megabytes
+
+  belongs_to :owner, class_name: "User"
+  belongs_to :updated_by, class_name: "User", optional: true
+  has_many :document_memberships, dependent: :delete_all
+  has_many :members, through: :document_memberships, source: :user
+  has_many :share_links, dependent: :delete_all
+
+  validates :title, presence: true, length: { maximum: 200 }
+  validates :content, presence: true
+  validate :content_is_document_json, if: :content_changed?
+
+  before_validation { self.content_updated_at ||= Time.current }
+
+  # :owner / :editor / :viewer / nil for a signed-in user and/or a share
+  # link token (a visitor without an account holds only the token). The
+  # strongest of all applicable grants wins.
+  def role_for(user: nil, token: nil)
+    roles = []
+    if user
+      return :owner if owner_id == user.id
+      m = document_memberships.find_by(user_id: user.id)
+      roles << m.role.to_sym if m
+    end
+    if token.present?
+      link = share_links.active.find_by(token: token)
+      roles << link.role.to_sym if link
+    end
+    roles.max_by { |r| ROLES.index(r.to_s) }
+  end
+
+  def self.role_at_least?(role, minimum)
+    role.present? && ROLES.index(role.to_s) >= ROLES.index(minimum.to_s)
+  end
+
+  # ---- edit lock --------------------------------------------------------
+  def locked_by_other?(holder)
+    lock_holder.present? && lock_holder != holder && lock_expires_at&.future?
+  end
+
+  # Take or refresh the lock for `holder`. Returns true when `holder` has it.
+  def acquire_lock!(holder, name)
+    with_lock do
+      return false if locked_by_other?(holder)
+      update_columns(lock_holder: holder, lock_holder_name: name, lock_expires_at: LOCK_TTL.from_now)
+      true
+    end
+  end
+
+  def release_lock!(holder)
+    with_lock do
+      update_columns(lock_holder: nil, lock_holder_name: nil, lock_expires_at: nil) if lock_holder == holder
+    end
+  end
+
+  def lock_info(holder)
+    { locked_by_other: locked_by_other?(holder), holder_name: (lock_holder_name if locked_by_other?(holder)) }
+  end
+
+  def meta_json(role:)
+    {
+      id:, title:, version:, role:,
+      owner_name: owner.name,
+      updated_at: content_updated_at.iso8601(3),
+      updated_by_name: updated_by&.name
+    }
+  end
+
+  private
+
+  def content_is_document_json
+    if content.bytesize > MAX_CONTENT_BYTES
+      errors.add(:content, "ist zu groß (max. #{MAX_CONTENT_BYTES / 1.megabyte} MB)")
+      return
+    end
+    parsed = JSON.parse(content)
+    errors.add(:content, "ist kein numolus-Dokument") unless parsed.is_a?(Hash) && parsed["sheets"].is_a?(Array)
+  rescue JSON::ParserError
+    errors.add(:content, "ist kein gültiges JSON")
+  end
+end