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>

main.go

@@ -146,6 +146,8 @@ func main() {
 	r.Get("/e/{slug}/slots", srv.handleSlotsPartial)
 	r.Post("/e/{slug}/claim", srv.handleClaim)
 	r.Delete("/e/{slug}/claim/{claimID}", srv.handleUnclaim)
+	r.Post("/e/{slug}/rsvp", srv.handleRsvp)
+	r.Delete("/e/{slug}/rsvp/{rsvpID}", srv.handleUnrsvp)
 
 	// SSE
 	r.Get("/e/{slug}/sse", srv.handleSSE)