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>

markdown.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"bytes"
+	"html/template"
+
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/renderer/html"
+)
+
+var mdRenderer = goldmark.New(
+	goldmark.WithRendererOptions(
+		html.WithHardWraps(),
+	),
+)
+
+func RenderMarkdown(input string) template.HTML {
+	var buf bytes.Buffer
+	if err := mdRenderer.Convert([]byte(input), &buf); err != nil {
+		return template.HTML(template.HTMLEscapeString(input))
+	}
+	return template.HTML(buf.String())
+}