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/models.go

@@ -17,14 +17,15 @@ type Claim struct {
 }
 
 type Event struct {
-	ID         int64
-	Slug       string
-	Title      string
-	Date       string
-	Time       string
-	Location   string
-	AdminToken string
-	CreatedAt  time.Time
+	ID          int64
+	Slug        string
+	Title       string
+	Date        string
+	Time        string
+	Location    string
+	AdminToken  string
+	Description string
+	CreatedAt   time.Time
 }
 
 type Rsvp struct {