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>

templates/event.html

@@ -26,6 +26,9 @@
     {{if .Event.Time}}<span>&#128338; {{.Event.Time}}</span>{{end}}
     {{if .Event.Location}}<span>&#128205; {{.Event.Location}}</span>{{end}}
   </div>
+  {{if .DescriptionHTML}}
+  <div class="event-description">{{.DescriptionHTML}}</div>
+  {{end}}
 </div>
 
 <div id="slots-container"
@@ -87,6 +90,17 @@
 {{end}}
 
 {{if .IsAdmin}}
+<div class="section-label" style="margin-top:40px">Admin: Description</div>
+<div class="claim-form-wrapper">
+  <form method="POST" action="/e/{{.Event.Slug}}/admin/{{.Event.AdminToken}}/description">
+    <div class="form-row">
+      <label>Event description (markdown supported)</label>
+      <textarea name="description" rows="5" style="border:var(--border-w) solid var(--border);background:var(--cream);padding:10px 14px;font-family:'Bricolage Grotesque',sans-serif;font-size:0.95rem;resize:vertical;outline:none;">{{.Event.Description}}</textarea>
+    </div>
+    <button class="btn-submit" type="submit">Save description</button>
+  </form>
+</div>
+
 <div class="section-label" style="margin-top:40px">Admin: Add slot</div>
 <div class="claim-form-wrapper">
   <form hx-post="/e/{{.Event.Slug}}/admin/{{.Event.AdminToken}}/slot"