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>

test/integration/documents_api_test.rb

@@ -0,0 +1,111 @@
+require "test_helper"
+
+class DocumentsApiTest < ActionDispatch::IntegrationTest
+  setup do
+    @anna = create_user("anna@example.com")
+    @bob = create_user("bob@example.com")
+    @doc = create_document(@anna)
+  end
+
+  def put_content(doc, base_version, headers: {})
+    patch "/api/documents/#{doc.id}", params: { content: DOC_JSON, base_version: }, headers:, as: :json
+  end
+
+  test "listing and creating need an account" do
+    get "/api/documents"
+    assert_response :unauthorized
+    post "/api/documents", params: { title: "x", content: DOC_JSON }, as: :json
+    assert_response :unauthorized
+  end
+
+  test "create (upload), list, read" do
+    sign_in @anna
+    post "/api/documents", params: { title: "Hochgeladen", content: DOC_JSON }, as: :json
+    assert_response :created
+    id = json.dig("document", "id")
+
+    get "/api/documents"
+    assert_equal [ id, @doc.id ].sort, json["documents"].map { _1["id"] }.sort
+
+    get "/api/documents/#{id}"
+    assert_equal DOC_JSON, json.dig("document", "content")
+    assert_equal "owner", json.dig("document", "role")
+  end
+
+  test "someone else's document is a 404" do
+    sign_in @bob
+    get "/api/documents/#{@doc.id}"
+    assert_response :not_found
+  end
+
+  test "save with version check; stale base_version is a conflict" do
+    sign_in @anna
+    put_content(@doc, 1)
+    assert_response :success
+    assert_equal 2, json.dig("document", "version")
+
+    put_content(@doc, 1)
+    assert_response :conflict
+    assert_equal 2, json["version"]
+  end
+
+  test "known_version returns metadata only when unchanged" do
+    sign_in @anna
+    get "/api/documents/#{@doc.id}", params: { known_version: 1 }
+    assert json["unchanged"]
+    assert_nil json.dig("document", "content")
+  end
+
+  test "edit lock blocks saves from another tab" do
+    sign_in @anna
+    post "/api/documents/#{@doc.id}/lock", headers: { "X-Client-Id" => "tab-a" }
+    assert json.dig("lock", "mine")
+
+    post "/api/documents/#{@doc.id}/lock", headers: { "X-Client-Id" => "tab-b" }
+    assert_not json.dig("lock", "mine")
+    put_content(@doc, 1, headers: { "X-Client-Id" => "tab-b" })
+    assert_response :locked
+
+    delete "/api/documents/#{@doc.id}/lock", headers: { "X-Client-Id" => "tab-a" }
+    put_content(@doc, 1, headers: { "X-Client-Id" => "tab-b" })
+    assert_response :success
+  end
+
+  test "share link without account: read with token; viewer cannot write" do
+    link = @doc.share_links.create!(role: :viewer, created_by: @anna)
+    headers = { "X-Share-Token" => link.token }
+
+    post "/api/documents/#{@doc.id}/join", params: { token: link.token }, as: :json
+    assert_equal "viewer", json.dig("document", "role")
+    get "/api/documents/#{@doc.id}", headers: headers
+    assert_response :success
+    put_content(@doc, 1, headers:)
+    assert_response :forbidden
+
+    get "/api/documents/#{@doc.id}", headers: { "X-Share-Token" => "wrong" }
+    assert_response :not_found
+  end
+
+  test "share link with account: joining adds it to the list" do
+    link = @doc.share_links.create!(role: :editor, created_by: @anna)
+    sign_in @bob
+    post "/api/documents/#{@doc.id}/join", params: { token: link.token }, as: :json
+    get "/api/documents"
+    assert_equal [ [ @doc.id, "editor" ] ], json["documents"].map { [ _1["id"], _1["role"] ] }
+    put_content(@doc, 1)
+    assert_response :success
+  end
+
+  test "owner deletes; a member only leaves" do
+    DocumentMembership.grant!(document: @doc, user: @bob, role: :editor, source: :invite)
+    sign_in @bob
+    delete "/api/documents/#{@doc.id}"
+    assert_response :no_content
+    assert Document.exists?(@doc.id)
+    assert_nil @doc.role_for(user: @bob)
+
+    sign_in @anna
+    delete "/api/documents/#{@doc.id}"
+    assert_not Document.exists?(@doc.id)
+  end
+end