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>

db/queries.sql

@@ -5,10 +5,13 @@ SELECT * FROM events WHERE slug = ?;
 SELECT * FROM events WHERE admin_token = ?;
 
 -- name: CreateEvent :one
-INSERT INTO events (slug, title, date, time, location, admin_token)
-VALUES (?, ?, ?, ?, ?, ?)
+INSERT INTO events (slug, title, date, time, location, admin_token, description)
+VALUES (?, ?, ?, ?, ?, ?, ?)
 RETURNING *;
 
+-- name: UpdateEventDescription :exec
+UPDATE events SET description = ? WHERE id = ?;
+
 -- name: UpdateEvent :exec
 UPDATE events SET title = ?, date = ?, time = ?, location = ? WHERE id = ?;