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/home.html

@@ -29,6 +29,10 @@
       <label>Location</label>
       <input type="text" name="location" placeholder="e.g. Long Meadow, entrance at 9th St">
     </div>
+    <div class="form-row">
+      <label>Description (optional, markdown supported)</label>
+      <textarea name="description" rows="4" placeholder="e.g. Bring your own blanket! We'll have a grill set up near the big oak tree." 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;"></textarea>
+    </div>
 
     <div class="section-label" style="margin:24px 0 16px">Slots</div>