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>

schema.sql

@@ -6,6 +6,7 @@ CREATE TABLE IF NOT EXISTS events (
     time       TEXT    NOT NULL,       -- e.g. "2:00 PM"
     location   TEXT    NOT NULL,
     admin_token TEXT   NOT NULL UNIQUE,
+    description TEXT    NOT NULL DEFAULT '',
     created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
 );