security: CORS preflight, session invalidation, CSRF on preview, symlink containment, and hardening

- Add OPTIONS /api/volumen/* for CORS preflight (S-22).
- Invalidate session in require_login! when user no longer exists (S-8).
- Add check_csrf! to POST /admin/preview (S-1).
- Use File.realpath for symlink-safe containment in media_file (S-2).
- Memoize Users#all with mtime-based invalidation (S-12).
- Document that empty permitted_hosts means allow-all (S-5).
- Warn and fall back to random when session_key is too short (S-20).

test/admin_test.rb

@@ -263,6 +263,25 @@ class AdminTest < Minitest::Test
     assert_includes last_response.body, "Username unchanged"
   end
 
+  def test_preview_requires_csrf
+    login
+    get "/admin/posts/new"
+    post "/admin/preview", "body" => "**hi**"
+    assert_equal 403, last_response.status
+  end
+
+  def test_orphaned_session_is_rejected
+    login
+    session_for = -> { rack_mock_session.cookie_jar["rack.session"].to_s }
+    refute_empty session_for.call, "expected a session cookie after login"
+
+    # Simulate a deleted user by clearing users.toml while keeping the cookie.
+    File.write(@users_path, "")
+    get "/admin/"
+    assert_equal 302, last_response.status
+    assert_equal "http://example.org/admin/login", last_response.location
+  end
+
   private
 
   def csrf(body)