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>

migrate.go

@@ -0,0 +1,17 @@
+package main
+
+import (
+	"database/sql"
+	"log"
+	"strings"
+)
+
+func runMigrations(database *sql.DB) {
+	_, err := database.Exec(`ALTER TABLE events ADD COLUMN description TEXT DEFAULT ''`)
+	if err != nil {
+		if strings.Contains(err.Error(), "duplicate column name") {
+			return
+		}
+		log.Printf("migration warning: %v", err)
+	}
+}