Add admin location editing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main.go

@@ -194,6 +194,7 @@ func main() {
 	// Admin
 	r.Get("/e/{slug}/admin/{token}", srv.handleAdmin)
 	r.Post("/e/{slug}/admin/{token}/description", srv.handleUpdateDescription)
+	r.Post("/e/{slug}/admin/{token}/location", srv.handleUpdateLocation)
 	r.Post("/e/{slug}/admin/{token}/slot", srv.handleCreateSlot)
 	r.Delete("/e/{slug}/admin/{token}/slot/{slotID}", srv.handleDeleteSlot)