The event detail template data set has_tag_21 to drive the
Kulturhauptdorf logo, but tag 21 is "Feste & Märkte", so the logo
showed on every market/festival event. Add is_kulturhauptdorf, which
checks for a tag named "Kulturhauptdorf" (case-insensitive), and keep
has_tag_21 as deprecated for backwards compatibility.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
