Add RSVP support and Open Graph meta tags

People can now RSVP without claiming a slot. OG tags show
event title, date/time/location, and headcount in link previews.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

db/models.go

@@ -27,6 +27,14 @@ type Event struct {
 	CreatedAt  time.Time
 }
 
+type Rsvp struct {
+	ID        int64
+	EventID   int64
+	Name      string
+	Note      string
+	CreatedAt time.Time
+}
+
 type Slot struct {
 	ID        int64
 	EventID   int64