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/queries.sql

@@ -51,3 +51,17 @@ DELETE FROM claims WHERE id = ?;
 
 -- name: CountClaimsBySlot :one
 SELECT COUNT(*) FROM claims WHERE slot_id = ?;
+
+-- name: ListRsvps :many
+SELECT * FROM rsvps WHERE event_id = ? ORDER BY created_at;
+
+-- name: CreateRsvp :one
+INSERT INTO rsvps (event_id, name, note)
+VALUES (?, ?, ?)
+RETURNING *;
+
+-- name: DeleteRsvp :exec
+DELETE FROM rsvps WHERE id = ?;
+
+-- name: CountRsvps :one
+SELECT COUNT(*) FROM rsvps WHERE event_id = ?;