Add markdown event descriptions for hosts to provide context

Hosts can now add a free-form description (with markdown rendering via
goldmark) when creating or editing events. Descriptions render safely
with no raw HTML passthrough. Includes ALTER TABLE migration for
existing databases.

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

main.go

@@ -104,6 +104,7 @@ func main() {
 	if _, err := database.Exec(schemaSQL); err != nil {
 		log.Fatal("schema init: ", err)
 	}
+	runMigrations(database)
 
 	funcMap := template.FuncMap{
 		"pct": func(count, max int64) int64 {
@@ -160,6 +161,7 @@ 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}/slot", srv.handleCreateSlot)
 	r.Delete("/e/{slug}/admin/{token}/slot/{slotID}", srv.handleDeleteSlot)